You are 100% correct. Actually the total weight of the sled + crew can go up to a certain amount of weight (1389lbs for the 4-men and 860lbs for the 2 men). When the crew is light, they add weight to the sled to get right on the weight limit since it goes down faster.
The advantage of a heavier crew is that you do not have to add much weight to the sled, which makes it easier to push at the start.
Now you don't see many small guys. Most are in the 220-240 range with some brakmen being in the 190-200 range from time to time (because they need sprinter-like speed and few 230lbs guys have it).
To make a bobsleigh team when you are light you need to be very strong and unusually (freaky) fast. Then you are used as the brakeman (the last one in, the one who pushes the longer) where your biggest contribution is the final acceleration before you jump in. Guys with more top speed at the 60m mark are best suited for that job.
When he made the team they had three very big guys (drivers and no.2 and 3) who were well into the 230s and 240s so the total weight of the crew was the same as every other crew.
But nowadays you see more and more big and fast athletes being brakemen. In north america you often see former college (or even pro, think Hershel Walker) RBs doing that job. In Canada the current brakeman for the Canada 1 team is Jesse Lumsden who until recently played (or still play) pro football in the CFL as a runningback and is about 225.