What are some reasons my cat is like this?

I have 2 cats. One is a pretty average size, the other is overweight. The overweight one eats about average, unless there is extra food in the other cats bowl, then he eats that as well. For a while now, the overweight one has been vomiting. It is not hairballs, it is normally undigested cat food. He doesn't seem to be loosing or gaining wait though. Now a couple times in the last 2 weeks, we have come home and found small amounts of liquid-y vomit in different places in the house. He is 12. We are taking him to the vet this week, but I just want to know what could be the problem.