What flash diffuser was likely used for this macro photo?

For macro, the softness of light depends mostly on the light source’s size relative to the subject. Because the subject is very small, even a simple speedlight can look very soft at close range.

Based on the clues in the discussion, this was likely made with an external Canon Speedlite and a small diffuser—possibly the built-in pull-out diffuser, the supplied diffuser, or a popular snap-on cap like a Sto-Fen. Reflections in similar images suggest some kind of rounded diffuser on the flash, but there’s no clear evidence of larger modifiers or side reflectors.

So the key point is: the soft look is probably due less to a huge modifier and more to macro scale plus a nearby diffused speedlight. A ring flash is another common macro option, but the answers suggest this image was likely lit with a standard external flash and simple diffuser rather than a ring flash.