Yay appears rather femme to me most of the time, and usually has small boobs. This appearance (4559 & 4600) they don't have noticeable boobs and a rather more masculine posture, even if covered by what I would read as a femme onesy. In the other embodiments of Yay, small boobs seem to be the norm, so it's not clear why a different chassis is used in this case, unless Yay is trying to appeal to Roko at some level as masculine, instead of femme.
We know Roko has dated masculines in the past, but I also infer Yay is trying to figure out human mating signals and using Roko as a target subject. However, Roko, being wrapped in her own problems, is oblivious to Yay's signalling.
It will be interesting to see where this all goes. And if Pintsize tries to date Roko again using his new chassis. Somehow a jealous Yay seems a thing one should not trifle with.