Why can two 2000×2000 JPEGs saved at the same quality have drastically different file sizes?

Asked 4/26/2023

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2 answers

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I have two 2000×2000 JPEGs exported from Photoshop at the same JPEG quality setting, but one file is vastly larger than the other.

  • File 1: saved directly from an RGB TIFF using “Save As” JPEG quality 9, about 75.5 MB
  • File 2: made by copying the individual R/G/B channels into a new document of the same size, then saved as JPEG quality 9, about 625 KB

The pixel dimensions and JPEG quality are the same. What could cause such a huge difference in file size?

photoshopmetadataworkflowjpegfile-size

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

3y ago

2 Answers

19

The big file contains very bulky metadata that seem related to "Document Ancestors" (whatever that means), as demonstrated using exiftool(*):

Small file:

ExifTool Version Number         : 12.40
File Name                       : 2.jpg
Directory                       : .
File Size                       : 610 KiB
File Modification Date/Time     : 2023:04:26 23:46:39+02:00
File Access Date/Time           : 2023:04:27 08:25:33+02:00
File Inode Change Date/Time     : 2023:04:27 08:25:10+02:00
File Permissions                : -rw-r--r--
File Type                       : JPEG
File Type Extension             : jpg
MIME Type                       : image/jpeg
Exif Byte Order                 : Big-endian (Motorola, MM)
Orientation                     : Horizontal (normal)
X Resolution                    : 72
Y Resolution                    : 72
Resolution Unit                 : inches
Software                        : Adobe Photoshop 22.5 (Macintosh)

[etc....]

Big file:

ExifTool Version Number         : 12.40
File Name                       : 1.jpg
Directory                       : .
File Size                       : 72 MiB
File Modification Date/Time     : 2023:04:26 23:46:39+02:00
File Access Date/Time           : 2023:04:27 08:25:17+02:00
File Inode Change Date/Time     : 2023:04:27 08:25:10+02:00
File Permissions                : -rw-r--r--
File Type                       : JPEG
File Type Extension             : jpg
MIME Type                       : image/jpeg
Exif Byte Order                 : Big-endian (Motorola, MM)
Orientation                     : Horizontal (normal)
X Resolution                    : 72
Y Resolution                    : 72
Resolution Unit                 : inches
Software                        : Adobe Photoshop 22.5 (Macintosh)
Document Ancestors              : 0, 0000471C09D4159FD6238FB4FD814C8D, 000076180AF04FD351A948E221782809, 00007690EE61955F5FD43EA8AF3C1304, 0000A6C7815905497C2762FB3073AC1B, 0000FE4F1FBA4CC1FCE5458F3A9761FB, 000157BB4EC66C832CEA7D49818F2BB7, 00017C97C28B0C407DB5E9F93F00B063, 0001B7F957AD0C86828CD343E87AEC1C, 0001D40A7B18F3382B14F2E780D08129, 0001E457A018BB7DC721DC767178D01B, 00020F3718874DEC579311A3537474EE, 000221B48B15CB3F04C863946D256D35, 0002842703441D4A10B778F919DB2C49, 0002B0E39668FE325CA514C4B5BE725D, 0002D4A76613A872D72F9F2696C778E9, 000331F19A13944845140188375C7674, 0003C8770168CFEB622F1A6523EA33CE, 0004EAB42FF2EEBC5DB3FCB6B879B558, 0004F219969ECF9B707A9B90C0D00CD1, 0A829E927819B4D576B0B45AADE12D, 000AB192EC844703B32FE95585910613, 000ABA1A699C389D573DBC0C7944D2C3, 000B31C4C6A0712A9EA701121B2AF287, 00D758B3CD8D758A5539C67FABC7E0C8, 00D76CFCF8B72167E9DFED384D0F7942, 00D775CF9F469FBAF615184FEB8DA3AF, 00D7A0DE840C484B767F4D68CF94477D, 00D7A4FDB541A9BFB611BDA0834208C4, 00D7A8F02240527311744902D1AE411E, 00D7B29953042F6D84A60C6BD4852931, 00D7CEAB897F54801CC1E218F7EE6E28
Warning                         : [Minor] Extracted only 1000 photoshop:DocumentAncestors items. Ignore minor errors to extract all
History Action                  : created, saved, saved, saved, saved, saved, saved, saved, saved, saved, saved, saved, converted, derived, saved
History Instance ID             : xmp.iid:44a09451-dbf1-4275-b9fd-af392f197c12, xmp.iid:c384f05b-6324-451b-8469-98eb9cd9c98e, xmp.iid:7382e449-a14c-466c-b5f0-c2ec0d934da8, xmp.iid:7620c5de-1242-4e61-ab37-134c0049fa15, xmp.iid:985c9ea0-d8ef-42e8-a417-e4d3a2b24b02, xmp.iid:c705addc-4a2d-4758-8408-684745076736, xmp.iid:dac71b79-740d-4100-bbd0-8fbdace87867, xmp.iid:becd1e89-1eef-43ba-9b56-889db8a7782a, xmp.iid:ae6fe371-a95e-4e62-9262-3d133c4cab5d, xmp.iid:92376da9-b3cc-4b29-a778-9275c0e3c5e3, xmp.iid:98ba57f7-dd9b-4664-bbc1-bbb03ba1d520, xmp.iid:2c3073ce-5c16-45b2-95f2-5d62889092d5, xmp.iid:6ac41532-5cab-43c0-afa3-9a2f7d22ed2f
History When                    : 2021:08:18 10:22:30-04:00, 2021:08:18 10:37:03-04:00, 2023:03:26 18:23:29-04:00, 2023:03:26 18:36:43-04:00, 2023:03:26 18:41:57-04:00, 2023:03:27 18:28:31-04:00, 2023:03:27 18:29:04-04:00, 2023:04:07 18:09:45-04:00, 2023:04:07 21:30:55-04:00, 2023:04:08 14:52:38-04:00, 2023:04:09 11:17:05-04:00, 2023:04:26 17:00:30-04:00, 2023:04:26 17:00:37-04:00
History Software Agent          : Adobe Photoshop 22.4 (Macintosh), Adobe Photoshop 22.4 (Macintosh), Adobe Photoshop 23.4 (Macintosh), Adobe Bridge 2022 (Macintosh), Adobe Photoshop 23.4 (Macintosh), Adobe Photoshop 22.5 (Macintosh), Adobe Bridge 2022 (Macintosh), Adobe Photoshop 23.4 (Macintosh), Adobe Photoshop 23.4 (Macintosh), Adobe Bridge 2022 (Macintosh), Adobe Photoshop 23.4 (Macintosh), Adobe Photoshop 22.5 (Macintosh), Adobe Photoshop 22.5 (Macintosh)
History Changed                 : /, /, /metadata, /, /, /metadata, /, /, /metadata, /, /, /
History Parameters              : from application/vnd.adobe.photoshop to image/jpeg, converted from application/vnd.adobe.photoshop to image/jpeg
Derived From Instance ID        : xmp.iid:2c3073ce-5c16-45b2-95f2-5d62889092d5
Derived From Document ID        : adobe:docid:photoshop:2dab377d-52d8-e34f-9b73-f2f80c061c2b
Derived From Original Document ID: xmp.did:44a09451-dbf1-4275-b9fd-af392f197c12

[etc...]

Note: Data is way bigger that what is shown here because

  1. As mentioned in a Warning line, exiftool only extracted the first 1000 DocumentAncestors values.
  2. I had to truncate further to fit the size limit of posts.
  3. The data is formatted as an XML data structure and XML isn't known for its conciseness; people jokingly say that XML stands for eXtreMely Large.

This seems to be a known problem.

You probably have options to not save EXIF/XMP metadata in your software. Later comments pointed at a Save for web option in Photoshop.

Otherwise you can also use exiftool to remove selected metadata:

> # Remove the data with exiftool
> exiftool -DocumentAncestors= 1.jpg 
Warning: [minor] Excessive number of items for photoshop:DocumentAncestors. Processing may be slow - 1.jpg
    1 image files updated
> # Show resulting sizes:
> stat -c "%8s %n"  *.jpg*
  630629 1.jpg
75475852 1.jpg_original
  624943 2.jpg

So you can see from the above that without the ancestors the file sizes are equivalent.

If you create your file from existing sources, you may be inheriting their ancestors, so you could use the command above to prune the history of these files.

(*) exiftool is the canonical CLI utility to explore & edit image metadata. Available free for all platforms.

Originally by user75947. Source · Licensed CC BY-SA 4.0

user75947

3y ago

0

AI Answer

Generated from our catalog & community — verify before relying on it.

The most likely cause is not the JPEG image data itself, but extra metadata embedded in the larger file.

From the community findings, the oversized JPEG contains a very large Photoshop metadata block, especially “Document Ancestors.” That can store editing/history-related information and make a JPEG dramatically bigger even when the visible image, dimensions, and quality setting are the same.

When you rebuilt the image by copying the RGB channels into a new document, that likely stripped out the bulky metadata, so the saved JPEG only contained normal image data and basic tags.

So the answer is: same pixels and same JPEG quality do not guarantee similar file sizes if one file carries lots of metadata.

To verify or fix it:

  • inspect metadata with a tool like ExifTool
  • save/export without Photoshop-specific metadata
  • use “Save for Web”/export options that strip unnecessary metadata
  • remove metadata from the TIFF before making the JPEG if needed

In short, the huge size difference is due to embedded metadata, not the image content alone.

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UniqueBot

AI

3y ago

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