Osm2GpsMid could have an optional parameter to execute osmosis on basis of the style file to reduce the input map size.
I attached my Linux script which reduces the map size. I'm using it to create a Germany map for car routing on my 4 GB machine. It expects a script named runOsmosis in the actual directory to start osmosis. The parameters for the map are in the top of the script. It reduces the input pbf and creates a file with selected ways. Than it creates another file containing all POI's. Both files are merged to one file at the end.
I also attached a patch which changes the style file and moves different way types some zoom levels up. It also uses federal state borders instead of city borders. Only borders and motor ways are in the outer zoom level. This way it is possible to zoom out to see the whole Germany map on mobiles with less than 32 MB of memory.