Thats not a very good situation. Don't be offended, but I'll try to express my "male-ish" opinion on this
Lets face the fact that for all who gathered here - feedism is an important part of their personality. And its a subconcious thing that may replace tradition sexual desire (at least for me thats true).
I believe that best relationships are those where there is a main important ingredient - affection to personalities of each other. And feedism thing is that exotic spice that makes that plain generic relationship dish delicious and desired. But as long its a sexual subconciousness drive - its blind for dignity and morality - its all wild.
You said that your former BBF had a lot in common with you, which might be an excuse for mixing things up in his head. I think in that case cheating with fatter (in that case just plain sexier in FA's mind) woman could be a hard-to-resist temptation to fulfull sexual desire for much fatter you, which can be done right now and just by going across the street.
I think you did good - not overreacted and asked for explanation. Because that might be just a failure to resist that temptation, a weakness, a mistake... But what he told you - that made it more clear, that he has a top priority on fatness of a woman he is with, than anything else. Its a relationship without a main ingredient, just water and spice - that relationship is unlikely to hold for long (I experienced something similar). I think that he becoming your fiancee - is a result of not thinking carefully enough of things and was driven by that feedist(sex) desire. Good that you haven't married - things could've been dirtier than that.
That's why I'm not rushing with getting married. I could just live with someone, but will consider marriage only if both of us will get along well with each other and truly understand and wish to be together "until death will separate them" (or something like this).