I'm going to take a guess that you are using automatic storage. (Not that this could happen otherwise...it is just easy to have this happen with automatic storage.)
The problem is most likely that your database reclaimed the space for itself but did not release the disk back to the operating system. This can be shown very easily by checking the High Water Mark for the tablespace.
Do a the following
db2 list tablespaces show detail
This will show you each tablespace and what it is using on disk.
Used pages is how many pages of disk the database is using. Comparing that against
total pages (the total claimed on disk) and the
High water mark (pages) will show you if you are "claiming" more than you actually need. (ie, low used pages, very high total pages and a High Water Mark close to the total pages).
To get rid of this unused space and return it to the operating system you would issue the following (under automatic storage):
db2 alter tablespace <tablespace name> reduce max.
db2 alter tablespace ts1 reduce max;
That will cause DB2 to lower the high water mark and release the unused disk back to the operating system. (Note you can only do this for regular and large tablespaces, not for system temporary, or user temporary tablespaces).
If you are using DMS without automatic storage you need to use a slightly different set of commands:
db2 alter tablespace <tablespace name> lower high water mark;
db2 alter tablespace reduce (<containter name> or [all containers] integer K|M|G or integer PERCENT);
db2 alter tablespace ts1 lower high water mark;
db2 alter tablespace reduce (all containers 500 M);
Where we work, we put this into some of our maintenance scripts so that we automatically run this after we do reorgs to make sure we reclaim disk space. In our case we use DB2 LUW 9.7 FP 4, so it doesn't hurt to double check Information Center for 9.5 to make sure you have access to the right information for your version.