Every time you create a database object, Access allocates more hard drive space, and your MDB grows. When you delete objects, however, the MDB size stays the same.
The actual process isn't as clear cut as what I've stated above, but that's what it boils down to. 'Compact' the database, and your size will shrink, sometimes dramatically.
Either way, you can expect your Access database won't fit on a floppy if you add any number of queries, forms, reports, etc.