Percival Graves is one of those people who's so dedicated to his work that he doesn't always take care of himself. This means grabbing some fries for lunch when there's an important case, maybe skipping dinner altogether to get paperwork done. He's not completely malnourished, but he weighs a bit less than he should.
When Grindelwald takes him prisoner, he doesn't have time to find out the recipe for gruel, and opts for feeding Graves whatever Grindelwald's having for dinner (which is much more nourishing than whatever Graves would usually be eating). When MACUSA comes to rescue Graves, he's actually gained weight from eating decent meals.