Compared to people, puppies grow incredibly fast. A puppy that weighs 200 grams at birth will weigh ten times that much by the time it is six weeks old! Even taking into account the different life expectancies and rough equivalents of dog age to person-age, that would be like a baby that weighed 3.6kg (8 lbs) at birth having grown to 36kg (80 lbs) by the age of 18 months!
By the time you take your puppy home at the age of 8 or 9 weeks, it will be roughly half its adult height. It will reach about half its adult weight at between four and five months of age (17-21 weeks). Growth continues to be very rapid until the puppy is about seven months old, at which point it begins to taper off. You should feed your puppy special puppy food until that point, to provide the extra nutrients needed for its rapid growth.
By the age of one year, your puppy will have almost reached its full height, but weight gain will continue, albeit at a much slower rate, for about another six months.
To estimate how heavy your puppy will be when fully grown, take the weight at 18 weeks (4 months and 1 week) and double it.