First, are you getting enough calories? I keep plugging this site, but its what I use: Sparkpeople.com. Free, easy to use. Make sure you're getting enough.
Two, eat lots of veggies! You're going to eat much more volume as a vegan than as a meat eater since meat is more dense and heavy. To eat more veggies in a condensed way, try a incorporating into smoothies.
Three, fiber! If you're eating whole foods, then this is not usually a problem. But if you're eating processed foods like cookies, white bread, etc, there won't be very much fiber in your food.
Make sure you have at least 1-2 servings of whole grains at every meal (e.g. rolled/steel-cut oats, brown rice, millet, quinoa, kasha, whole wheat bread...) They keep you full and satisfied, and help keep your blood sugar level, keeping cravings at bay.