I think you're underestimating the complexity of building scalable file syncing across multiple platforms that pretty much just works from the end users' perspectives.
Nowhere did i say, "Dropbox should be the same size as 37 Signals." I merely suggested that they would have been better off trying to be the right size for their revenue profile (which is probably larger than 37 Signals, but certainly much smaller than their current growth target of Google/Facebook/Amazon/etc. size).