> a few example images that were set very very small, often ~2cm and usually in a grid

Journals sometimes require the authors to pay "page charges" of sometimes hundreds of dollars per page. I know of some (relatively high impact) journals in image processing related fields that require this. If the author is not working under a grant, then there's not much incentive to add to the page length.

Of course, this is insane that this is even a thing in the digital era, and it feels very mobbish. Idk. Maybe there's valid reason that I'm not aware.