Disagree with this as this is all dependant upon body type. That's the only thing that would explain being a Boo L and a Gitman small since I'm a BoO small and a Gitman small and they both fit slim. There's no way I could fit an XS or XXS if it even existed in Gitman. I posted side by side comparisons in a recent purchases thread about a month back. The hem on the BoO; however, is definately cleaner and has less hip flare than the Gitman, but then again there's no doubt the Gitman is design for tucking but still work for going untucked.
i'm a small in BoO and a small in gitman. gitman fits a bit bigger, but smaller in teh neck.