Sugimori designed by himself all the first 251 pokémon, working really hard and under a lot of pressure both times.
In gen 3 he started leading a whole staff that varies a little every new generation.
You are just experiencing a difference in style, which is actually needed if you are going to keep the series for ever or we will have over 1000 monsters that end up looking the same. Cloyster face everywhere, Nidoking spikes everywhere, fat pink things everywhere.
Sujimori still does the final art though. Adding his style to the designs made by the staff. Also, consider that 1st gen mons were highly changed when the anime animators drew them on screen. Such animators are not in charge of the anime anymore and the new ones do not have any autority on pokémon design.
End of Story.
