They didn't. My parents understood that not everyone wants to eat the same amount of everything, so they didn't make a big deal about it. I was small-framed kid (had a small-framed mother), so my mother understood that everyone doesn't want a giant plate-load of food. We didn't have junk food (other than as an occasional treat), so my mother knew that whatever we ate was good food. She wasn't worried about quantity, and she'd always say, "I wouldn't want someone to try to make me eat when I don't want to eat, so I wouldn't do that to anyone else."
The only thing my mother would ever say about food to anyone was, "Take as much as you know you'll eat, but please don't take load up your plate and then not eat it, because that's a waste."