Your stomach is a muscular bag that has a capacity of roughly 30 ounces of liquid and solid matter. People who eat large meals regularly may stretch that capacity to some degree. And those who go on steep diets may actually shrink their stomachs.
Suffice it to say that zombies are not likely to join Weight Watchers anytime soon. Zombies are the competitive eaters of the monster world, inclined to stuff themselves at each and every brain buffet they can find.
There are some problems with this style of consumption, and it has nothing to do with a zombie watching his figure. Instead, it has a lot to do with wherever that food winds up.
Because zombies aren't known for their fully functioning bodies, they may very well have gaps in their digestive system somewhere between their mouths and anuses. Gaping holes in this route make it impossible for zombies to derive any sort of benefit from their meals at all.
Of course, because so few parts of zombies' bodies work, there's a good chance that the brains they eat will just hit a dead end (pun intended) where the esophagus hits the stomach. As the zombie dines on more and more brains, the undigested glop of chewy stuff would just ferment, bubbling gases and eventually bursting through the walls of the zombie's stomach.