Just trying to understand, did he say he watched the porn because he liked big bums?
Best case scenario, he likes bums, he liked yours. Still likes yours. But in the course of watching porn got hooked on a particular 'aesthetic' ......because guess what, in recent years fashion for "thin thick" (tiny waist, big legs and bum) has been following the porn aesthetic of ultra feminine look which almost cartoon, plastic and implausible . Also I think if anyone did a study into how porn is filmed looking at stuff like camera work, you'll probably find there is now a more obvious trend for camera angles focusing on arses. I cant be bothered to hypothesise why. Its probably pretty obvious.
The thing is, men who watch porn have their preferences shaped by it. He may well have just blurted out what he did because he was on some level as unhappy with the situation as you.
Plus, its perfectly possible for two statements to be correct even when they seem to contradict.
"I like big bums" and
"I like small bums"
simply means "i like bums"
The problem only occurs if he said "I don't like....."
Maybe he just blurted it out because he was confused, ashamed, concerned he was obsessed, realised he was ruining his relationship.
The other factor I think with people's sexuality, and preferences is that so much of it is always hidden to other people, sometimes even to the person themselves. So its quite likely that most of us have fantasies we ourselves are slightly ashamed of, or fear the other person might shame us for. An example of this is fetishes, another example would be men choosing a relationship with a slim attractive blonde woman of similar age, whilst simultaneously having a porn preference for older brunettes who are obese.
If I were you I'd not let his "preference" dictate my happiness. The best way to pick up your self esteem is to tell him in no uncertain terms "this is me, small bum, I'm not exercising or changing, like it or lump it" my guess is he will lump it because his porn preferences are not his actual active conscious preferences.
As for the rest, money etc,...maybe you need to get some advice and do some thinking.