I have used JOSM to download a section of the OSM along a railway corridor doing 4 saves into the same layer, then updated the data and saved the lot as an .osm file. I did it this way because the area was too big for one download using JOSM.

But only a portion is made into a Garmin map for my eTrex, the last of the 4 sections saved.

The dropbox image shows the size as JOSM sees the osm file when reloaded and the gmapsupp.img is the red rectangle  https://www.dropbox.com/s/hfeo5zi48wxapl8/JOSM_save.png?dl=0  

Is this the expected result? I wanted all the .osm saved file to be made into the gmapsupp.img

I have tried this process several times with the same result.

Here is the command used from iMac El Capitan 10.11.6 with Java8 Update 101.