What causes rapid weight gain???

About 8 months ago I moved out of my mom's apartment where I lived for several years with her and her cat Romeo. Since I've moved out he has gained ALOT of weight. I'm not sure how many pounds he's gained because I'm not sure what he weighed before I left, but I can tell just by looking at him he has gained a very significant amount of weight. I feel like every time I go over there to visit he looks even bigger than the last time I saw him. I'm conserned for him because he looks very obese. I know he doesn't eat excessively. My mom feeds him about 3/4 cup of food every day and he doesn't even finish it. I think she feeds him Purina Adult Formula. He is around 8 years old. Does anyone know what can cause such rapid weight gain?

I beleive diabetes can cause rapid weight gain/loss.
Are you sure he isn't being fed too much? Afterall, you're not there to really see.

I think the first thing I would do is cut back on the dry food and get him on a wet food diet. Dry food has a lot of carbs and wet food had the moisture that cats really need.

Bijou was getting quite heavy (16.2 lbs) and since I've been feeding more of a wet diet (Merrick) he has leaned up and I think he looks much healthier.

As the above poster also said, a vet visit would be a good idea to rule out any medical issues.

A vet VISIT ASAP is needed... it could just be a senior ingesting to many calories but please VET VISIT