It's the chunks, and then some. A hash file is going to have empty space all over it. When you insert something, the hash algorithm picks a more-or-less random block to store it in. Only if that block happens to be full already will the file grow.

OTOH, I've heard of hash algorithms that have problems when too many keys map to the same block. That has the effect of limiting your maximum file size, but the maximum size isn't something easy to compute. When in doubt, dump the values back out of the file to make sure they're really in there.