Hi Beat,
I am almost certain it is not your code. At the moment I encounter a lot of memory leaks in the last patchlevels ( 9, 9HF, 10,10HF ). So a lot of memory is allocated and not released ( at least the most ). I stepped over that when mass updating items -> it is about 0.3 mb per item. Since then I made some tests and could reproduce the behaviour for many business objects. Also SBO client seems to be affected -> within half a minute of just opening forms from main menu the memory usage is going up like crazy and even after closing most is still allocated.
In result : it is not your fault and you are not alone...