Obviously I only know you from a few words but you talk a lot about how much she loves you but very little about how much you love her. She's cool with your preference because she doesn't want to lose you, but at some point she'll get jealous — rightfully.
You have to look into your heart and figure out how much you really love her, and whether you're gonna be able to deal with her size in the long run. You say her love for you is rare, which is true, but being loved is very powerful and it inspires loyalty, which is not the same as love.
If you want something different and you're staying with her because you don't want to break her heart then you have to let her go. Rip the band-aid off and deal with the fall-out. You'll break her heart anyway, eventually, getting wrapped up in fat porn and distancing yourself from her, or cheating. Don't waste 4, 8, 12 years of her life (and yours) if she's not what you need.