Some weeks back, when I was having problems downloading map updates (well, that's a surprise, isn't it!!!) I was trying (and failing) to download the updates with both Garmin Express and MapUpdater. I seem to remember that the size of the download that Garmin Express was saying needed downloading was smaller than what MapUpdater flagged up. I cannot see any reason why, but that is what it was saying. So, just perhaps, maybe garmin Express is indeed better able to squeeze the update onto the device.
I was just eventually happy to get the downloads downloaded, so never got to the bottom of the discrepancy
I was just eventually happy to get the downloads downloaded, so never got to the bottom of the discrepancy