Here is where it starts getting interesting. A snapshot initially takes up little to no space. As changes are made to the source database the snapshot grows in size. In fact the snapshot is the size of all of the pages changed in the source database since the creation of the snapshot. Basically as a page is changed in the source database a copy of the original page is made and stored in the snapshot, but only the first time. (Note: The files used to store these pages are called sparse files.) This means that if you change the same page over and over again it will only be written to the snapshot once. It then logically follows that the largest a snapshot can get is the size of the source database at the time the snapshot was taken. Since most of the time we change a very small portion of the database at any given point in time this means that snapshots tend to be much smaller than the source database. In fact you could load millions of rows into the source database (assuming they are mostly/all in new pages) and it will have little to no effect on the size of the snapshot.
My favorite use of database snapshots was so developers could test their changes in QA and then revert back to a pre-snapshot environment. That way, they could preserve data for future runs.