It doesn't match his previous sprites because it's a different art style all together.
It has been three-odd years since the first so it would make sense if he changed physically to match his new title as emperor, still, i agree that it feels wrong i wont lie...
If we ever get a "Adastra 2" of any sort i would definitely like to see him regain a bit of his older built. It felt integral to the whole "Warm/Grounded emperor" we saw so much of in the first.
Then again as that other person noted, it could just be different perspectives of him, of course Marco would see Amicus as more of a comfort character whilst Scipeo would see him as more of a "Leader."
On a more positive note... The new sprites and art style look awesome! Virginia's whole character feels so much more complete with her new design, same goes for Neferu.