How to make my pitbull bigger

First let me say that I am not interested in dog fighting I love my boy way to much to harm him but I want him bigger so that he looks good. And when I say bigger I mean muscle my dogs eats 1 cup 3 times a day but he isn't getting bigger I walk him...

constently and run with him and im starting to worry that he might have a problem and yes he has no worms any advice

some dogs just arnt genetically muscly...just like some humans arnt.
but just liek humans you need to run with him more often then, thats their working out so run with him and play more active gameslike catch with a frisbee and stuff.

Yeah he could just be petite in structure thus making him unable to gain to much muscle mass, just like not all german shephards are big I had one and she was very petite. But he probably just is small framed for a pit. Try getting him to swim that will work all muscles and do a lot of exercises that force his shoulder muscles to work and play time activities.

What food are you feeding him?

Well, size is totally dependent on genes...how big were his parents? How old is he?

Feed him a premium kibble, and get him plenty of exercise, and he will muscle up...if he's younger than 16 or 18 months old, he simply hasn't filled in, yet...they do their upwards growing in the first year...and the next 6 to 12 months filling out.

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A lot of pitbulls compete in pulling competitions. They are strapped to a harness with a certain amount of weight on it and they have to pull it a certain distance. Maybe you can make a harness with weight that you could get your dog to pull and get him to bulk up. A lot of dogs love this kind of exercise!