It is hard, I had to do it like a year ago. My oldest (they're only a year apart) was underweight for a while, so I changed the wet food I give her in the morning and she gained another pound or so. They're both small-framed, so I don't know if they'll ever weigh more than 10 pounds in their lives.  My friend has a Maine Coon, that dude is a TANK. He weighs like 20 pounds, but Maines are traditionally big cats. I want one.