database physical storage

hai...

 	I have observed that the size of data (for example size of 
table in a database) stored by database  system on physical storage is 
more than actual datasize.It seems  database is storing data in some 
format.For example i have created  a table with data 15MB considering each 
INTEGER of 4bytes.But after loading  this data into database i have 
observed that database system is storing this table in 45MB memory.So can 
anybody tell me what is the reason to use so much memory? Is there any 
relation between actual dataset size and size of data stored on physical 
storage system.

thanks in anticipation

regards,
-- 
  N Praveen Kumar

 	Imagination is more important than knowledge...
                                 --Albert Einstein