Reason:

They're the same thing, as far as I can tell. If one's meant to differentiate thighs large from muscle vs body fat, we have the "quads" tag and the chubby tag group that can just be used in addition to the tag.

Unfortunately, I can't seem to make a proper alias request either way. I think it's because "thunder_thighs" is aliased to "big_thighs", while "large_thighs" and "thicc" are aliased to "thick_thighs".