It doesn't seem like a desirable evolutionary trait. I'm sure there are some cases of, basically, deformity, and if you don't actually have a thigh gap maybe if the area is wet for a long time it might start chafing a bit. Never heard of any normal sized people having inner thigh issues or premature wearing in that area. I think that fats permanently ruin their legs so they get that inner-bow shape and always walk on the inside of their feet with the heel kinda rolling in (you know, the fatty walk). Even when they lose weight, the damage is done.