My gf is 5'7" and runs size 2 with a size 0 in some brands. What she calls her "skinny" weight is 121lbs (and she looks a little too skinny then) and her more normal weight is 125-128lbs.
There's absolutely nothing wrong with having a thicker frame and needing a larger size either. But let's get back to the girl in the original post. Just exactly how is she only "big boned"? Look at her legs -- are her femurs 5" in diameter?
We need a reality check on that girl:
1) She's "all muscle" so she must be a 900lb squatter
or
2) She's "big boned" and has femurs that are 5" across
or the real issue...
3) She's got a thick (but relatively firm) layer of fat on her legs that's at least 3" thick.
I'm waiting on the next theory. Perhaps her veins, arteries and nerve bundles are just all 2" across each. Maybe she's just "big veined".