While it's entirely possible to ingest too many calories as either a vegetarian or a vegan, I think it's probably more difficult as a vegan -- but not impossible as there are plenty of calorically dense vegan options -- including what most of us would think of as "junk" -- or at least foods that should constitute a small part of one's diet -- cookies made without dairy products or chips are vegan but calorically dense.
However, I think if someone is combining a vegan diet with some aspect of nutritional awareness, one will probably naturally wind up consuming fewer calories than one opting for an omnivorous diet because vegan food choices tend to be less calorically dense than other food choices so one is going to get satiated on fewer calories.
For myself, a vegetarian diet would be difficult to control my weight on since I rely on lean protein (as well as vegetables/fruits) as part of my food plan. I find that I need to strictly control even complex carbohydrates to control calories and satiation. I do think if one has been eating large quantities of high fat meat/fish and then opts for vegetarian and doesn't replace with cheese or high fat preparation of fat, perhaps. But I do better with a meal of lean chicken/fish and veggies rather than even a Cooking Light meal centered around whole grains.
Some days I pray for Silence, Some days I pray for Soul,
Some days I just pray to the God of Sex and Drums and Rock 'N' Roll.