In my opinion Ymir is more like a tomboy, not a butch. We shouldn't see only her breast dimension.. But, after all, in this couple there isn't a "strong" one, 'cause they're both weak in a certain sense. I can't write more because of spoiler, but I think that. About the muscles.. uhm, you're right, Bluetrode, for me they MUST have more developed muscles, even Christa, who seems a lady so candy and cute.