You're assuming the intelligent species is anywhere close to the size of human beings. And that their generations are anything like our generations. You're assuming that they eat like we do, or weigh as much as we do.
Just because a human generation ship would have to be large, it doesn't mean some other species' generation ship would have needed to be.
To account for the size of the various UFOs that have been seen, if those accounts are to be taken at face value, we don't seem to be dealing with space smurfs.
Just because a human generation ship would have to be large, it doesn't mean some other species' generation ship would have needed to be.