As a bigger girl, I love that the Sims are versatile, and we can make our sims look (to a point) exactly like ourselves.
The fat sims are great; the skinny Sim would be fine if the tendons weren't so out and apparent. I know a LOT of VERY VERY skinny people, and usually I can see collarbones, ribs, MAYBE spines sometimes, but what looks off to me about the very thin sim is how the muscles lay out. I don't think it has anything to do with the weight, just the detail in the muscles and how the shadows are on the skin.
And to everyone that is getting offended -- fat people have been trying to stick up for themselves against skinny people for so long that I think its just going to far the other way; now SKINNY is the bad thing, while fat people are being more and more accepted. They're both A-OK, ok. They just are. As long as you're eating correctly, and getting your 30 minutes a day 6 days a week, 8 hours or sleep, and plenty of water.