Jackson is not 20lbs, but he's about 18lbs so I'll reply anyway.
He used to maintain at 16lbs, but I'm thinking he still had some filling out to do after 3, because he's not gotten any fatter, but the last 3x we've went to the vet he hit about 18.5lbs. I was just thinking scale was off, but yeah, I guess he's my almost 20lb yorkie
I find I still say '16lbs' a lot though because I got so used to it.
Anywho, on most days, he eats 1/2 cup per day of kibble (Acana, Go! at the moment) but he also gets some canned food (Weruva) and treats (chewy Buddy Biscuits, and Zuke's) plus the occasional bully stick, or chew.
I do up it to 2/3 cup of kibble per day if he's spent a lot of time swimming and seems extra hungry.