I think a fair bit of Garmin Express for Mac users will appreciate that Garmin clarifies if the Mac version is supposed to be able to "sync" pre-cached satellite data to Fenix/Fenix 2
No, it didn't shrink. The EPO.BIN supplied by the iOS Connect App was 4000+ bytes. The EPO.BIN supplied by the curl is 64000+ bytes. I have no idea why they're so dissimilar. It's possible I didn't wait around long enough for the Bluetooth connection to transfer the file, I suppose. Anyway, at the moment I have the EPO.BIN from curl, piped through a little perl script I threw together to cut out the 3 byte segments, installed on my watch. That seems to work on my Fenix so far; today I turned on the GPS several times and satellite acquisition was much faster than normal. I don't know how long it takes for the information in EPO.BIN to become stale, though.
The reason I looked into it in the first place was that the unmodified EPO.BIN from the curl just wasn't working on my Fenix 1. I also noticed the same initial 3 byte difference from the EPO.DAT (and I assume the subsequent 3 byte differences as well) you can download from mediatek itself. But the file from mediatek is even larger, ~ 275 kbytes. I haven't tried to find any documentation on the structure of the EPO file yet to see how it's laid out or why the differences between the three.