Not to get too technical but here is a side by side of the same art style of the second game, so very early on in the series where they already seem to have decided to make Morrigan bigger. Now since I'm having to cite sources for my arguments, Game:Night Warriors: Darkstalkers' Revenge, release date: March 3, 1995
Is this the biggest boob difference of all time? No. Am I saying that Morrigan is BTFOing Felicia so hard that she makes her tits look like a prepubescent child in compassion? No. Am I saying that there are versions of Felicia that beat Morrigan by a bit? Yes. But there's a very clear difference to me, and it's not that close.
Also in answer to your question about if this pic >>2256952
is a cross over, no it's from a remake of the first two games called Darkstalkers Resurrection. And for the record while I'm pained to say it, even if it was a cross-over it's art was presumably paid close attention to. It's not like Capcom was overseeing this remake of a series and said "Fuck it make Morrigan and Felicia look like whatever you want lol"
It's such a strange argument to me, like here's a silly example: Can Sakurai just make all the characters in his game look however he wants? He gave Samus Aran a noticeable boob reduction in Ultimate, much closer to her Zero Mission design than her Other M design. He couldn't do that without express permission, in fact it was probably a push from Nintendo. There's artistic license and then there's source material company oversight, I don't know of many spin-offs or crossovers or whatever where the art team went way outside of the original character's appearance.