Plant sources may be low in calories and high in fiber which could help lose weight but there are some sources that say, vegetarian diet can also make a person fat. I partly believe this since the low calories on vegetables are compensated by high calorie ingredients such as oil and starches and cheeses.

I don't think it would cause you to be obese. But as I have always said. Anything in excess is harmful eventhough plants and veggies are healthy still they can't replace all the food groups you are avoiding to take like protein perhaps.