While I also don't like muscular women in theory, a lean lady that works on her body can be very sexy. Once you really love a person the looks anyway doesn't matter that much anymore (unless she gains a ton of weight). My SO also gained a bit of weight at some stage of our marriage, barely noticed it because it doesn't really matter. I also went through stages of gaining a crap load of weight and then losing it all after an op and being skinny (soos in sieklik maer), and she stuck by me through it all.
So try to not be to fixated on looks, girl 2 can also change quickly, it can take something small as a sudden illness and she gains a lot of weight, or loses a lot, will you then also not feel attracted to her anymore?
Without this other woman in the picture, would you still have wanted out? If not, don't be to hasty. My SO have both asked each other for a divorce multiple times over the last almost 9 years, but in the end that was just really bad dips, some lasting for many months, but if both are willing to work on it you should at least try.
While you're saying it very harshly, you are not wrong. Agree with all that you said here. Marriage isn't easy.