Okay, at this point I have to admit that I assumed that cheating by adding extra weight is actually a thing here. My belief was based solely on the movie
Cool Runnings, where the John Candy character had been suspended for doing just that previous to the movie. In retrospect, using a John Candy movie as the basis for my belief might not have been the wisest move (much as I like John Candy). But who new that a movie might include something that's not actually true?
So I did a bit of research (limited to the net because I'm lazy). There's a page
here that discusses cheating in bobsled, but there's no mention of using extra weight as a method to do so. The wikipedia page
here states that there's a maximum weight for the bobsled which includes the racers themselves (which addresses the question above of the racers being physically big - it doesn't really matter). If the sled is underweight (because of lighter racers, e.g.) they add weights to the sled to take it up to the maximum. It also states that "[t]he bobsleighs themselves are designed to be as light as possible to allow dynamic positioning of mass through the turns of the bobsleigh course", which doesn't make much sense to me. If the whole package is allowed to weight X, what does it matter of 20% of that is the sled and 80% the racers, or vice-versa?
There is a reddit asking the same question as I did
here. Some interesting stuff there, but it seems that the answer is still uncertain.
But I couldn't find anything about actually cheating by adding weight to the sled - and from what I read about the pre- and post-race inspections (similar to the post-race weighing of handicapped race horses) - it wouldn't be easy. There seem to be easier ways to cheat (which are actually used).
So it all may be moot anyway.