I thought vegetarians were thinner simply because in general, they were younger. I guess we'll have to wait until all the pretty young veggies turn 50.
Look at the extra people in the background in movies and TV shows from 1970's. It seems to me that people in general were a lot skinnier in the meat-eater days before veg caught on. Then again, in the early 1970's, we didn't have bad oils, high fructose corn syrup, pink slime, dwarf hybrid wheat, and everybody smoked too. Our food and lifestyles have changed so fast that time comparisons are nearly impossible.
5'0" female, 44 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 115(!) on SAD chocolate, potato chips, and stress. Currently keeping food tracker.
I (try to) follow by-the-book primal as advocated by Mark Sisson, except for whey powder and a bit of cream. I advocate a two-month strict adjustment for newbies. But everybody is different and should tweak Primal to their own needs.