Outside of just using the debug menu, there’s a lot of traits you can combine to increase weight.
In order of calc weight (how much each trait affects character weight): Gluttonous, Comfort Eater, Drunkard, Hashishiyah, Lazy, Greedy, Calm, Trusting, Gregarious, Patient, Arrogant and Ambitious are the vanilla traits you can use, as well as the Reveller Lifestyle traits (obtained from repeated feasts).
There’s also the appetite traits from the mod which are congenital, so try marrying characters to others with increased appetites to pass those traits on to their children.
The opposites of all of these traits also make characters lose weight so bear that in mind.
Hosting feasts will also increase the weight of characters that attend, and if you’re a feudal/clan kingdom/empire (and have the Royal Court DLC), try to keep the court expenditures as high as possible, especially the food. The tooltips in game show you how each expenditure affects weight gain.
Character’s weights update once per year on their birthday, so if you see events where you’re meant to gain X weight that won’t be added until your birthday.
There’s also other mods that can make certain traits more common within dynasties, such as House Traditions and Personality Dynasty Legacies if you want to really hone in on certain traits.