When calorie intake is consistently increased, a vegetarian will gain weight same as a meat eater  It takes a surplus of 3,500 calories - or 500 calories a day - to gain one pound, roughly 1/2 kilogram. But vegetarianism notwithstanding, it may be difficult to gain one pound week-after-week because the body makes metabolic adjustments to the increased calorie load and you might get sick of over-eating. Consider starting with an extra 250 calories a day and make adjustments based on your outcome. Also, be sure to exercise to stimulate the appetite and eat enough protein. Good protein sources for vegetarians include nuts and seeds, lentils and beans, soy products. cereals, dairy products, and eggs when eaten.