Nokia 6233 - internal memory leak?

Hello,
I did several file operations with my Nokia 6233, e.g. I copied
several ringtone files forth and back from my computer via USB cable.
The phone's internal memory seems to become less and less.

What I then tried was:
I deleted a folder with about 50 files from the internal memory.
Then I copied it back. But there was not enough space left for all the
files (which had been in memory before!?).
Repeating this step leads to less memory left.

Please, can anybody help me with this strange phenomenon?

Thank you very much in advance!

Regards,
Ben