There was a wide variance in the body types. What tends to happen in these types of situations, I call it TIL Syndrome, people hear one small trivia fact and then expand on that, in good faith, to places where it does not apply.

Gladiators were likely to be pudgy. The majority of gladiatorial "styles" were built around a somewhat thick body type. Don't think of a Monty Python Meaning of Life style horrifyingly obese man rolled out into the arena demanding to be fed fresh slaves, but imagine someone that primarily weight lifts. Notice how they tend to have bellies, meaty thighs, kind of like NFL Linebackers.

It was easier to achieve this body type, they would be strong, they wouldn't be doing a lot of "cardio" or fat burning exercises, and on top of that if they were cut it would create a spectacle of blood without necessarily risking the death of a prized, expensive fighter. Everyone basically wins when the fat man gets cut.

Generally speaking, ancient soldiers were going to be leaner. They walked for long periods of time and had a variety of work outs/exercises that were available to them. Marius's men would carry all their gear on them, acting as excellent long distance weight training, they would build and assemble camps, there was an emphasis on running in Greek sporting events.

The soldiers were not likely to look much at all like the gladiators, since they were two wildly different schools of "battle", if we want to use that term, and their life styles were so wildly different.

Can you link the thread you are referring to?