Database size after SGEN

Dear Friends

Version : SM 7.1

DB : SQL server 2008 R3

OS : Windows 2008

I have installed solman 7.1. When I completed my installation the size of database was 35 GB. Then I ran SGEN and the db size become 55 GB. I checked if I could get some space by using database shrink but it shows only few MB's are free.

Can any have idea why this much space it has occupied?

Thanks

Atul