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RSE and FRLG look different, just look at Red and Leaf's overworld sprites when they appear in Emerald and Brendan and May's sprites when they appear in FRLG. The proportions are totally different. The same is true when comparing overworld sprites in Gen V to Gen IV, especially when comparing returning characters like Cynthia or at the PWT. Some people might think it's just because it's on stronger hardware, but Gen III is way more bright and saturated than Gen II. The change in Sugimori's art style between Gen II and III was also pretty noticeable at the time, character proportions were much slimmer. To be fair, he did switch from watercolors on paper to digital during this time too, and the Gen II human proportions everyone tends to look at were from 1997, not 1999. Later artwork for the Gen II male protagonist is sleeker and more expressive.
When comparing Sugimori's art he made for RG to his Blue art, the Blue art (despite being posed far more dynamically) also has a far more extreme focus on highlights in the watercoloring while the Gen I colors are more flat.
A large percentage of Gen V characters were illustrated by Yusuke Ohmura whose line work, expressions, shadows, etc. all differ to Sugimori's. The Rocket admins in HGSS were done by Takao Unno, who also did the art for the first Pokémon Ranger and later directed B2W2. In Gen VI, all the NPC characters are done by an assortment of artists, while the overworld follows a sort of chibi 3D look. Alola tries to go for proportions closer to the actual artwork, and there are many tropical colors and motifs while the characters are more cartoonish (nearly all illustrated by Sugimori). LGPE's art was done by Megumi Mizutani.
There has always been different art direction depending on the game. Even HGSS tries to use deeper colors than DPPt with its cool colors.
