Here's a few era-specific headcanons for Roy being fat that range from humorous to tragic
Titans (1999) Roy taking advantage of the fact that the Titans kitchen is always fully stocked as one less bill to worry about and gradually putting on quite an amount of weight after finally noticing just how much he can eat with steady access to food
Outsiders (2003) Roy discovering he's begun stress eating after the shit he's endured on this time, ranging from being shot in the chest multiple times, Lian's abduction, and finding out he was being manipulated by Deathstroke. A mix of staying closer to Lian more AND eating more.
Titans (2008) Roy discovers there's lingering effects of when Raven transferred the powers of her brothers in the Titans, and as a result of having briefly been "Gluttony" he's been stuffing his face more without meaning too while Donna's dating life as has been reinvigorated and Vic's snapping at every little thing Gar does
(No I will not do one for Titans 2010)
A common element to these (well, aside for 2008) is that Roy is basically replacing one vice with another. People overly demonize weight gain and using food as a vice, as if it's anywhere near as life threatening as heroin. Roy's whole thing is that he's trying his best with the crappy hand he's been dealt, and if he turns to a large pizza to help with his stress, there's nothing wrong with that. He's put in the work, let him find some form of solace.
