I've been vegan for about 3 weeks - not a long time, I know. But I have seriously lost almost 10 pounds - and I wasn't huge or anything to start with (I was 5'5", about 140 lbs, now I'm down to 132 lbs.)
I work out daily and have since April 2005. I run, lift weights, do yoga, elliptical, stair master, etc. As an omni, I got more toned but really didn't lose any weight. I followed the Self Magazine Diet Club religiously for 3 months (must record everything you eat, all the exercise, and follow their calorie recommendations) - and I didn't lose a freakin' pound. Not one.
I went vegan and suddenly I've lost 10 lbs over these 3 weeks. I am convinced it's because I'm no longer eating dairy or meat. I'm eating tons of veggies/salads, soy/rice milk, fruit, spelt cereal for breakfast, roasted veggies/stir frys, etc. I am definitely eating a good amount - but I'm dropping pounds!
Of course, everyone is different - I'm just saying that, for me, I tried losing weight as an omni and got nowhere - now I'm a vegan and the weight is dropping!