Why can a lower-resolution image have a larger file size than a higher-resolution one?
Asked 4/2/2020
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2 answers
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I have two image files where the lower-resolution image is larger on disk than the higher-resolution one. For example, one image is 3200×2192 pixels and 5.86 MB, while another is 6000×4000 pixels but only 4.55 MB. How can that happen?
More generally, what factors affect image file size besides pixel dimensions? I’ve also seen two images with the same pixel dimensions end up with very different file sizes.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
28
The reasons to have difference in size can be (and most of them are related to image compression):
- Amount of details in the image. Save flat colour image and another with several colours and you will see the difference
- Number of colours. Related to above, but if you have more colours and lossy compression you may have bigger image (as size)
- Level of JPEG compression. This can change significantly the size
- Amount of meta information in the image
- High ISO can also result of bigger images (with the same other parameters) because higher noise (caused by high ISO) add "details" to the image.
And at the end size of the image (megapixels) is important but not so much
Originally by user34947. Source · Licensed CC BY-SA 4.0
user34947
6y ago
0
Generated from our catalog & community — verify before relying on it.
Pixel dimensions are only one factor in file size. The biggest reason is usually compression.
With JPEGs, file size depends heavily on:
- compression/quality setting used when saving
- how much detail or texture the image contains
- blur vs sharpness (blur compresses better)
- image noise, such as from high ISO, which adds random detail and makes compression less efficient
- metadata stored in the file
So a larger image can still have a smaller file if it was saved with stronger compression, has smoother areas, or contains less fine detail. A smaller image can be larger if it was saved at very high JPEG quality or has lots of detail/noise.
Even two photos with the same pixel dimensions can differ a lot in size if one is noisier or more detailed than the other. For example, high-ISO images often produce bigger JPEGs because the noise gives the compressor more random information to store.
In short: resolution affects file size, but compression settings and image content often matter more.
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