Is it okay if a cat eats some dog food?

I have two cats and two dogs. The cats are fed both wet food (about 3oz for both of them both morning and evening) and about a cup of dry cat food for free feeding per day. (I'm going to avoid using brand names, but they're good quality and contain no by-products or grains) My dogs eat dry food only (They're both over 85 lbs) and are fed morning and evening. One of my cats has taken to eating the dog's food in addition to his food. When the dogs are fed he will sometimes eat out of their bowls right alongside them. I try to shoo him away, but unless I physically hold him back while the dogs are eating he'll find his way back to their bowls. I'm afraid he's getting a little chubby because of this. I know cats need the protein and taurine from their food, which he should be getting in a good amount since he has no trouble eating his own food too. Other than the fact he's gaining some weight (which I'm keeping an eye on) is there any problem with the fact he's eating dog food?

(My other cat is a little thin but still within normal weight range and has no interest in eating dog food)