I'm semi-active in the modding area on the discord, and part of the issue of adding new body part sizes is that the png doesn't distort to match the size, it can't change at all. If your clothing item is too large, to accommodate massive titties, then it's just cut off, and the limiters for that are actually pretty narrow for some reason.
So new body part sizes would require effectively a rehash of how clothing in general is handled, to incorporate squash and stretch for fitting the clothes to new, enormous jugs (or in my preference, cutting-board chestlets)