The problem one always has with Labradors is that they are extremely veracious eaters. That is, if you were to allow them to eat until they were full, they would continue to eat and eat. This is why Labradors more than most carry a pot-belly. 

You did not say why you think this might not be enough food. All dogs at this age may look slightly undernourished, being tall and slender rather than muscular. I would recommend that you change brand of food to one which displays clearly the recommended quantity of food to give.

Weekly Expert Pet Advice
with Dr. James Glover, Resident Expert
taken from Pet Peoples Place