As if zombies weren't trouble enough, sometimes life (or undeath) throws fat zombies into the mix.
Since zombies are reanimated corpses, and since there'd likely be obese people amongst the casualties in a zombie outbreak, this is sensible. It's often used, though, to combine the disgusting factor of extreme corpulence with the hideousness of zombification. Also, from a practical film making standpoint, "fat" zombies allow the stuntman to wear padded clothing and take a beating from the heroes.
For whatever reason, fat zombies tend often be kamikaze attackers in video games, usually detonating in a shower of gibs and toxic fluids upon termination. Eeeeew.
Other times in video games, they tend to fill the role of bulletsponges or battering rams, somehow more durable than even "regular" zombies, due to their extra mass meaning they need more effort to shoot or chop up.
In non-video game media, though, they tend to not be too much different from other zombies.
See also Fat Bastard, Kevlard. For another way zombies can stand out in a crowd, see Incongruously Dressed Zombie.
Not to be confused with The Load, who's only dead weight from a figurative/narrative standpoint.
Lampshaded in Zombieland, where Columbus notes that the fat ones were the first to go for not being able to outrun zombies. He makes "Cardio" his number one rule. Unlike most other examples though, these aren't really any different from a regular zombie, aside from their weight.
3rd Edition Dungeons & Dragons has a monster in their Monster Manual 2 that's called a "Famine Spirit." It's the reanimated body and spirit of an individual who died of starvation, and returns to eat in undeath what they couldn't eat in life. About the only thing this creature doesn't eat are other undead. It even comes complete with a vorpal bite (that is, a bite that can sever your head from your body.)
House of the Dead features the Stone Wall, bullet-sponge variety. Frequently chuck barrels at you and hold open gates for their rotting, angry friends.
House of the Dead 4 also features the massive boss enemy ironically called Temperance.
Overkill also has the Lobber, the boss of level 5 ("The Fetid Waters"), who, like the Boomer pictured up top, is not only fat but covered in swollen, pustulant boil-tumors. Though thankfully not shown, it's implied his projectiles are pus-filled boils he's tearing off of his body and throwing at you.
Killing Floor has the Bloat, which walks up slowly, vomits acid as a close-range attack, and explodes when killed. They do soak up a few shots before dying, however.
The Boomer from Left 4 Dead (pictured). His special attack is to vomit a strange bile on surviving humans, which summons a sizable swarm of the speedy regular Infected to attack the afflicted humans. The vomit also messes up the Survivor's vision temporarily. When destroyed, the Boomer explodes in a shower of bile with an effect essentially the same as its vomit for anyone too close to it. Ironically, it has the least health (one shot offs them) of any of the special Infected as the result of being an undead goo balloon. Notably, the Boomer appears to be stretched out and inflated, implying it wasn't so obese before it got infected. Oh, and its "outie" bellybutton is really just its guts bursting out.
Although more similar in origin to the monster from Frankenstein (pieced together from multiple corpses), the Abominations from the Warcraftfranchise fit this description. Massive (definitely bigger than a human), hideous (putrid and decaying with internal organs visible), and misshapen (extra limbs abound), they fill a heavy infantry role for Undead forces. They also can emit poisonous fumes.
The Flash game series The Last Stand has lots of fat zombies, of the Kevlard variety. Since the only thing that separates them from regular zeds is their health (some can sprint just as easily as the thin ones), they fill in the role of Elite Mooks, along with the ones in riot gear.
Resident Evil 6 brings us Whoppers and the even bigger Whopper Supremes, both able to soak up massive amounts of damage, and can kill you by falling on top of you after re-killing them.
One of the Ancient's zombies fit this mold in Eternal Darkness. In this case it's stated to be due to internal gasses, as described in the Real Life section below.
Dead Space has the Pregnant, a swollen Necromorph that carries a hoard of mini-Necromorphs called Swarmers in its stomach. Shooting it in the center of mass results in the Swarmers getting loose and attacking the player.
Myth: The Fallen Lords featured an enemy that was a bloated corpse that would explode when it got close enough or was killed and temporary paralyze everyone within it's radius. Using archers killing them at a distance you could avoid the nasty side effects while possibly setting off a chain reaction if there were more than one close enough together.
Fat zombies exist in Dead Rising, which take slightly more hits to die. They all resemble Ronald, a notable scrappy fat survivor.
Silent Hill 3 has the insane cancers, which look like mutated grotesque fat "things".
Big Boo from the Super Mario Bros. series are giant ghosts, which could be considered to be another type of obese undead.
Metal Slug 3 and 4 feature fat zombies among regular zombies. They usually take more hits to die than a regular zombie (although less than a soldier zombie, curiously).
Fat zombies in Doom 3 can predictably take more lead before going down. Surprisingly enough, a great number of them carry wrenches to use as clubs. Also, they are the only type that is found being chewed on by other mooks.
While we haven't yet encountered any reanimated corpses, inanimate corpses— particularly in hot, wet climates— often swell up with gas. A tropical zombie could very easily turn into one of these (and even explode.)
The phenomenon of corpse bloat is discussed in excruciating detail in Stiff, and the book Vampires, Burial and Death: Folklore and Reality mentions a mass grave of chicken carcasses where the gases actually forced the bodies back out of the ground! Read it here.
Dead whales are classed as dangerous cargo because they have a tendency to violently explode.
William the Conqueror was very fat, and died of a large cyst developed from a bruised intestine. When they finally boxed him, he burst. The stink was so bad it cleared out the church his funeral was in.
King Henry VIII weighed 28 stone at death and thus required an oversized coffin. The corpse exploded in Syon Abbey while en route to Windsor, splattering blood and guts everywhere, and dogs were found licking up the remains.