I just sorta eyeball it too, but I know that they get enough. They actually get alot more than enough
They get 1 1/2 - 2 cups of leafy greens (plus the other vegs) 2 times a day. So in all reality each pig gets 1 1/2 - 2 cups of a leafy greens a day, divided into 2 meals.
Also auburnmare- I don't think that veggies will make them gain wieght, unless maybe if you are feeding them too often or too much (ie. feeding 5-10 baby carrots per meal, fruits everyday, etc.)