From what I've seen of other people going vegetarian for a short time they often lose weight as they have a smaller variety in diet causing them to get bored of the food options more regularly and thus eating less overall.

People who stay vegetarian for longer than those first few months tend to stop losing weight and eating normally again because they figure out ways to expand the diet beyond meat substitutes(all of the tofu, beans and cheese, but that's just 3 things, there's a limited number of combinations that feel different enough from others) and start eating a more sustainable diet without trying to just eat normal food but swapping meat for something else.