I think this is actually a lot more common than you would believe. I am a fairly active member of the 'fat acceptance' community, and there is an odd dynamic in place in a lot of relationships.
You see a lot of bodybuilder types going after bigger girls, finding the curves and shape, softness etc enticing. It might be the contrast, it might be an admiration of the indulgence that the women obviously give in to.
Who knows? But it isn't weird at all, or even that rare. tbh, it boils down to the fact that we don't control what we find to be beautiful, we only know the urge to connect with what has become sexualized for us.
I myself dig a curvy girl, I don't like the rail thin type.