that’s not really why there’s a limit for upload size. the reason is more that iNaturalist isn’t trying to be an image hosting service. even if you upload a large file, iNat is going to resize it down to no larger than 2048px along the longest size. so uploading anything larger than that is just wasting data bandwidth and server processing time to resize the image.
there’s more related discussion in a similar feature request that was declined: https://forum.inaturalist.org/t/image-upload-size-limit-warning-should-be-dropped-as-images-are-automatically-resized-anyway/39645/18
you shouldn’t reduce the size of the files by reducing the quality. rather, you should resize them to no larger than 2048px along the longest side since that is what iNat will do anyway upon receiving any image larger than that. there are lots of programs available to help you quickly resize a bunch of files. see https://forum.inaturalist.org/t/image-upload-size-limit-warning-should-be-dropped-as-images-are-automatically-resized-anyway/39645/4.