Board Thread:Questions and Answers/@comment-25322628-20170113025521/@comment-25322628-20170116032509

Honestly, sometimes size doesn't matter half the time. It would for the Colossal Titan, but if you can use your lesser size to your advantage against a larger adversary, your chances of prevailing increase.

Maybe Ymir's advantage won't just be her size, but whatever ability she got from devouring Marcel.