Access dosent like deleting stuff when you delete it.
Lets say X represents DATA and Y represents structure info (queries, forms, etc..) and let's say O is blank data
Here's a sample file
YYYYYYXXXXXXXXYYXXXXXXXXYYYYYXXXXYYYYYYYYYXXYYYXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXYYXXXXXXXXXXYYYYYXXXXXXXXXXYYYYYYYYYYYYYYYYYXXXYYYXYYXXXXYYYXYYXYY
so if you delete most your data (some info is retained, but not much)
you're left with:
YYYYYY00000000YY00000000YYYYY0000YYYYYYYYY00YYY0000000000000000000000000000000000000YYXXXXXXXXXXYYYYY00000000000YYYYYYYYYYYYYYYYY000YYY0YY0000YYY0YYXYY
see where the wasted space is?
this technique saves time when modifying the database, especially when the .mdb file acts like a db server in itself.
Going to Tools->Database Tools->Compact and repair database would leave you with:
YYYYYYYYYYYYYYYYYYYYYYYYYYYXXXXXXXXXXYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYXYY
I hope this helped