What’s interesting about the Greeks is that in Homers “Odyssey” allusions were being made to the younger generation being weaker than the soldiers that fought at Troy. Odysseus for example rips a wheel off of a chariot to use as a discus and they very test Penelope has for the suitors involves them being able to string Odysseus bow. The theme of the next generation being a bit softer seems to be a constant across time and geography. In the waning days of the Ottoman Empire the old guard would lament that the youngsters couldn’t string a bow and Samurai began lamenting the newer generation and it’s recounted in the Hagakure. As for training the Greeks were mad grapplers and rowers. As for the Romans my understanding about them was that it was superior tactics that enabled them to defeat Goths and Gauls that were all bigger than them. Wasn’t there some research lately done that showed Gladiators were actually quite fat to add to the spectacle?
That's what we call a topos - in this case glorifying your past to serve as an example for future generations.
As for the gladiators, that newsline was based on the gladiator cemetary in Ephesos. However, it appears the anthropologists have jumped to conclusions there: the only evidence they had from the bones is that the gladiators were apparently eating mostly plants (you can tell that from the C to N ratio), and they concluded that they must have been fat because of it (I suspect they were both Atkins followers, or it simply made for a good headline). No other signs of obesity were present, at least none were published. There is also Galenos who criticized the predominantly plant-based diet, since he thought it would not lead to ideal muscle development as seen in the Greek athletes. This was again interpreted quite bravely as "purposefully getting fat to be protected against stabs", and I can't help but wonder whether these people know anything about nutrition, exercise physiology or the use of bladed weapons at all.
There have also been other attempts to interpret the gladiator's diet as "vegetarian" or even "almost vegan", both of which miss the point in my opinion.
The main thing to consider, in my opinion, is that fighting with weapons mainly requires TONS of drilling, therefore a huge volume. While strength plays a role in the actual encounter (especially with the big scutum), weight classes are less relevant because of the different armaturae and the fact that the arena is much bigger than a boxing ring. That means that training for it basically was predominantly an endurance sport, probably 6-10 hours per day. And if you look at the diet of endurance athletes, you'll find that the vast majority of them eats mostly plants (they'll usually still end up with enough protein, simply because 6000-8.000 kcal of ANYTHING this side of pure oil or sugar tends to have enough protein for most intents and purposes; just 5000 calories from barley and beans (2:1) will give you over 200 g of protein per day, and most likely it was served at least with a bit of cheese, bacon and the occasional egg, since all surviving recipies from the time call for that). Tell them to change their diet "because of science", and they'll refuse, simply because what they do works for them. The number of people who get fat on such a routine, on the other hand, is quite small. The only examples I can think of - kushti wrestlers, Sumotori, Bökh competitors (basically everyone who wrestles without a weight class) - all emphasize one or several protein sources in addition to carbs.
These anthropologists also seem unaware that the gladiators were basically eating the same food as the Roman legionnaires, so I assume those must have been fat as well...
PS: For reference, I teach Archaeology at the university of Zurich these days, and I used to do gladiator stuntwork (for the lack of a better word) for a certain gentleman called Marcus Junkelmann for nine years (in addition to competing in Freestyle and Greco wrestling, hence the interest in kushti etc.).