Using the osm2mp method gives a final file of 119.8MB where the direct conversion gives a final file of 103.7MB
Hmm, did it only include 90% of the roads?
I don't know how you'd tell!!
I think others are seeing similar shrinkage and nobody, yet, has reported any missing roads.
This is using the splitter.jar, right? What might be happening: The splitter's output contains a bounding box, and all data within a somewhat larger box. If mkgmap reads that .osm-file, it will clip all data to the bounding box. osm2mp on the other hand will disregard the bounding box and convert all data to .mp. If you then compile the .mp- file, you should get that extra safety margin also, which may well amount to 10%.
Sub-optimal. The best route home for me is to use 2 motorways (think the 2 shortest sides of a triangle) whereas the Garmin has always tried to send me along the hypoteneuse. Unfortunately whilst this is shorter this is an A road which goes through three middle sized towns and therefore has traffic lights B roads etc all the way along so takes considerably longer.
You could try putting some special tags on this A-road: traffic_lights=many or expected_speed=low (these could be autogenerated in a preprocessing step). Then edit the "lines" file to give them a lower speed and/or class. Cheers Robert