In the November/December ’08 issue of Archeology magazine, paleo-pathologist Karl Grossschmidt talks about bone analyses of ancient Roman gladiator skeletons. He says they ate a mostly vegetarian diet of barley and beans.
He goes on to say:
Packing in the carbs also packed on the pounds. “Gladiators needed subcutaneous fat,” Grossschmidt explains. “A fat cushion protects you from cut wounds and shields nerves and blood vessels in a fight.” Not only would a lean gladiator have been dead meat, he would have made for a bad show. Surface wounds “look more spectacular,” says Grossschmidt. “If I get wounded but just in the fatty layer, I can fight on,” he adds. “It doesn’t hurt much, and it looks great for the spectators.”
I have trouble believing this. I've worked in Africa in communities where people do hard physical farm labor all day. These are people who are poor, but have enough to eat to the point of fullness. They survive on a diet that is 95% grains, legumes, and vegetables. They're lean (ripped, in some cases) but not hugely muscular.
So were Gladiators really fat? Do we have historical accounts describing their appearance? Are they shown as fat in artwork or statues?
We know from their bone structure that they were overly muscled compared to their contemporaries and they had an almost exclusively vegetarian diet. Livy called them Hordearii - "barley men." They also took calcium supplements in the form of either charred plant or bone ash, which lead to extremely high calcium content in the bones. The strontium content was also extremely high, lending credence to the vegetarian argument.
So we know they were elite athletes who trained for strength and they were certainly of a more muscular build. As for fat, it's harder to say. Practically it makes sense for them to be of a thicker build than a more slender one for all the reasons given in your post.
The hypothesis is drawn from a lot of evidence but may overreach a little. They were extremely well cared for and ate extremely carb-rich diets. An extra layer of fat would offer very little in the way of protection of a clean strike though. Galen also stated that the diet high in barley and beans "made the flesh soft," further strengthening the argument. It's important here to note that Galen thought this was a bad idea and the fat was a hindrance, not an advantage. A fatter gladiator would obviously tire more quickly, so any advantages to fat need to be measured against this weakness.
The artistic interpretations generally show well-defined muscles. However, they also show a more barrel-chested and wide-hipped figure than the "idealized" beautiful depiction at the time. That isn't always the case, some were quite lean, and their "type" of gladiator they fought as may have depended on their physical build.
Finally, it's important to remember there were a ton of these gladiator schools. Grossschmidt's study was well done on a group of 60+ gladiator graves at Ephesus which was a major city at the time. The problem is the bodies were from one arena and one time period. To draw conclusions about a several hundred year tradition that spanned the entire Mediterranean from a sample size like this is problematic.
I think a fair conclusion to draw from current evidence is that these were well muscled and heavier built than average men, but for actual subcutaneous fat it may have varied by time period or even specific schools. I want to say they probably had a bit of fat on them, but that would be only from Galen's commentary and would run into the same problem as Grossschmidt.
For sources other than Grossschmidt's study there are dozens of good ones, but check out The Warrior Diet by Ori Hofmekler and Diana Holtzberg, Republican Roman Army 200-104 BC by Nicholas Sekunda, Gladiators 100BC-AD200 by Stephen Wisdom.