so is thick a combi of fat muscle and good genes or what?
And bone structure. I think "thick" is as you mention above, a specific bone structure, a good amount of muscle mass, and bodyfat levels - approx as low as 15% to as high as 20 +%- that are very even, giving a women a very curvy feminine shape: small waste, round ass, etc. Many mistake fat for thick, and it aint the same thing. This is "thick" in the true sense of the term:
All things being equal, I will take a gal who 15-20lbs over weight vs 15-20lbs underweight any day.