So yeah I just smile and think about how “Private Security” could’ve gone in the direction of having Will be out of shape and unsuited to help Dick as proof of how unhealthy it was with him looking at Will for a Wally substitute.
And instead Will has absolutely no problem keeping up with Dick and the others and can easily suit back up as Red Arrow when needed, and he’s able to recognize what Dick’s doing through a name slip specifically BECAUSE he has no issue fighting alongside him.
And so Will’s belly is simply used as a downplayed example of how he’s in a better place physically, mentally, and emotionally after what a malnourished twig he was in the second season, and “Private Security” highlighted he’s capable of returning as Red Arrow for future episodes.