If you never compact your database, your database will retain the "wasted" space and continue to consume additional space to accommodate additional data. As an example, if you delete all of the records from a 1MB database and then add 2MB of data, your database will be in excess of 3MB. The 1MB of space, which was allocated to the deleted records, is held by the database as wasted space, which only serves to corrupt your database and fragment your data.

To avoid generating excess amounts of wasted space and corrupting your database, you must compact your database periodically. 

The Compact Utility reduces the physical size of your database by making an exact duplicate of the database while also ridding the database of any excess space created by deleting and modifying data. Compacting is the only way to reduce the size of your database. The Compact Utility restructures table records and objects, and then stores them in successive memory blocks, eliminating wasted space. The utility also updates table statistics to reflect database characteristics of the restructured data. The more additions, deletions and modifications performed on your database, the more often they should be compacted