But… Why? Since the default (apple) emoji are 64x64 and emoji one (i think) are 74x74, it wouldn’t make more sense to have them resized to the current emoji scheme base size?
In all seriousness, tho, I’m guessing that the -thumbnail operation is a bad choice. It focuses filesize over everything else, and while this may be fine for actual thumbnails, it’s not desireable for emojis (which are small enough anyways).