Thanks for the detailed analysis. It was pretty easy to find the problem now (a memory leak). It explains the behavior you describe: At startup the maps are loaded into memory. I measured it on the desktop, the loading leads to an additional memory consumption of 220 MB. Because of the memory leak, after the installation of a new map (causing another parsing), another 220 MB are allocated and so on.
RC 2 will have the memory leak fixed and contains the workaround for the too detailed polygons. This brings the memory consumption down to 66 MB with all monav maps installed (in comparison it's 52 MB without any monav map).
I'll test it on the N900 when I get the device back, but I'm pretty sure it will behave much better in this (rather extreme) setup. RC 2 is scheduled for 2011-07-06; I'm not sure yet if I'll do an intermediate release before.
Thanks for getting a female contribution to the voice of marble contest, much appreciated