I lost about 5 or 6kg after becoming vegan. I am still not sure whether this was due to the sudden absence of all animal products, or because I wasn't taking in enough calories. It was probably a bit of both. I would stir-fry huge amounts of vegetables, and at first I didn't realise that even though the quantity was big and I felt full after eating, the calorific content wasn't high enough. Once my thinking expanded beyond 'vegan = greens! Lots of greens!' and I started to make seemingly little changes like adding walnuts to soup, my weight became stable.