This will probably be entirely unhelpful to most of you but I just felt like chiming in on various things -
I'm 4'10" and about 130 (so chubby, yes) and I have a very small frame - small shoulders.
I actually wear the K12 size. I find that it is the perfect length (I have a very short torso and prefer that my shirts come just to my waist, NOT hips.) and the perfect width in the shoulders [most regular size people stuff, is way too wide in the shoulders, making them bunch up as if I were wearing shoulder pads. Ugh.]
K12 is a little snug in the chest for me (I'm a 38C-Dish), but not uncomfortably so.
By contrast, the Women's Large , which I have one shirt in, is much tighter in the waist, a tiny bit snugger in the chest, slightly too wide in the shoulders, and about 5 inches too long. Which makes it bunch up unflatteringly at my waist. I haven't tried the WXL, but I'm sure it would be even more annoyingly longer without much corresponding give in the bust/waist, while also continuing to be way too wide in the shoulders.
Anyway, blabbering aside, my tip to the shorter/smaller ladies out there is to try the K12, especially if you don't mind the shorter length shirts.
Hey woot - how about a K14? Same proportions as the other kids tees, but a bit longer/wider in chest/wider at hem?