Here's few things to look for if you're worried that the heifer isn't milking enought for the calf:
1. If his flanks are sucked in and guant looking, and he never seems to "fill" out during the day.
2. When the calf nurses, does he suck one teat, then the next, etc. etc. in rapid succession?
3. Does it seem like every time you check him, the little guy is up and sucking?
These are rather general, but I've noticed that calves that exhibit this behaviour are getting short changed in the grocery department by their mother.
The fact that your calf is lazing around might be an indication that his belly is nice and full, especially if he's looking bright and alert.
I agree with Dun that a small udder doesn't neccessarily mean a lack of milk, as some cows produce a very high fat milk with a small bag, and their calves grow like weeds in the garden.
Take care and best of luck.