Meat eaters do have an advantage. The most obvious one to me is the protein issue...let's say a vegetarian is meeting their protein requirements for building muscle (i.e. 1.5-2g/lb). If you're 150lbs, you wouldn't want to be getting *all* 300g of your protein from whey and casein shakes...ugh, fvck no. In fact some vegetarians may not even drink whey and casein because of it's derivation from animal products, much less egg protein.
So the other sources for vegetarians are soy protein, which have quite low BV (biological value) and are pretty crap proteins (bad amino acid profile). Not to mention soy's potential hormonal effects..
So, getting decent protein is more of a problem..
Also, many vegetarian diets lack fat, which is very important for hormone control and hence muscle growth. This can be put right, however, by drinking olive oil, flaxseed oil, etc..
So there are disadvantages, but that doesn't mean you can't grow muscle as a veggie.
-shaun