The African elephant is the largest living land animal and weighs up to 5,400 kg. As vegetarians, elephants require much food, sometimes consuming more than 225 kg of plant matter a day.
Rhinos eat grass, leaves, twigs and shrubs. White rhinos eat grass. Black rhinos eat bushes, twigs, branches and even thorn bushes.
A Hippopotamus has a diet in the wild consisting of soft short grass, and other plants, and fruits that have fallen to the ground.
The three animals mentioned above are all mammals. They eat what I would call a vegetarian diet and they gain weight.
Humans are mammals also, so I would say yes, you could gain weight by eating raw food.