There seems to be a lot of variation in the quality of the harness included in that kit (not that I'm knocking the kit, but the same harness is available in a bunch of other forms...such as the "Latigo Leather Harness"). I've always loved them, but from talking to Epiphora, I know I've been lucky to not have some of the issues she did with her one.
I play with some bigger women, and we are able to trade this harness back and forth (my hips are in the 45" range, hardly petite, but still much smaller than I've seen it go). I have also watched very skinny women use this harness happily.
I'd advise you to look into getting the "Latigo Leather Harness", which is the same as the one in this kit, but with a better backing pad, and buy a medium silk to use with it. You'll spend less, and both pieces can easily be replaced and improved upon once you figure out how well strapping on works for you and your partner. If you're a bigger girl, you want to make sure the dildo you select has some length to it because part of the length near the base of the dildo likely won't be useable for penetration when you get your bodies lined up.
And actually I just noticed that your post was in relation to the new curve kit. I don't know how well the curve will work, as I have never used one, but based on its dimensions I'd say its a great place to start for strap on sex.