Here’s the first possibility that comes to mind: when users are uploading images over a certain resolution (2000x2000 or close to it) iNaturalist resizes them to fit within that size range, and probably applies compression like you’d expect- let’s call that the “damselfy” scenario to fit with your examples.
When users upload images they’ve already resized to a resolution below that threshold, iNaturalist does not resize them. Consequently the file may come through with whatever compression level it was post-processed with- even if they’ve used low compression resulting in a large file. Let’s call that the “grass” scenario.
Many users who use a post-processing workflow do resize their photos in terms of resolution before uploading to avoid dumping that processing onto iNat’s servers- I always resize to within 2000x2000. But it would be easy for someone to mistakenly assume it’s desirable to select the highest-quality compression level while doing that. I set a higher compression to keep filesizes well under 1mb. I remember there was a topic a little while ago about cropping and resizing before upload to preserve detail on the target organism, but I don’t recall compression being brought up as an additional consideration.