I think the real reason for RGB's weirdness relative to literally every other game is more because they like...didn't have a unified art style at that point.One also has to remind that RGB, Yellow and several gen 2 sprites were very unnatural in proportions in order to show head or main features of the Pokemon, which obviously was not possible to do well in 2d sprites.
Sugimori mentioned in his book that he drew artwork of all the pokemon after the spritework was done, because guide books needed it. The sprites were just sort of thrown in there, is the feeling i got, and probably didn't have a lot of passes resulting in things like ivysaur
Just sort of...being really lumpy and off.
Yellow & most of GSC didn't have this problem. I think a lot of poses from Yellow are directly based on anime stock art and GSC just plain probably had more experience on their dot artists and art pipeline.
I guess, anyway, they rarely ever talk about this kind of thing.