Having a bigger clip will give you more bullets Having a bigger mag will give you more room for your bullets I'm pretty sure they got it right when they made the "more bullets" perk call clip size. I guess they could patch in a magazine size perk for you, but I fail to see how having a big empty mag sticking off your gun will help. Unless your compensating for something.
You'll notice the magazine size does not change with the endorsement, so the term is accurate. The AmmoMule commercial is just that, a commercial. The sponsors like to embellish their products some. You know the real AmmoMule only has enough space for two slices of bacon?!