reduce memory usage of "Internet" and "contacts" The DiskUsage app shows 14,8MB data used for "Internet" and 11,5MB used for contacts.

Is there a way to reduce memory usage? 

I know I can delete Internet data using the general app settings but I fear this will also delete all saved passwords and I'd like to avoid that.

As for the contacts: In the global account settings (sync) I use only my two Google accounts and weather. I had Facebook sync activated while back, though, and it import a number of contact photos, which are still visible.

Any chance to reduce memory usage without removing too much useful data?
Assuming by "Internet" it means the default browser, then you should be able to go to Browser Settings to delete History and other cached data (press Menu button on the Browser, then select More, then Settings).

As for Contacts, if your address book are synced to your Gmail Address Book, you can Clear Data in the Manage Applications. After a short while, the Contact app will resync to Gmail Address Book hopefully without the Facebook contacts.