The maps aren't compressed (however, some resource data inside the maps are, but even still, they aren't decompressed until those resources are actually needed at runtime and only in memory)

The issue is that when the engine sees that there is a HHD it will copy the map you're trying to load, plus a few shared maps (which total out to about a gig or so themselves IIRC) over to a temp. cache drive (this is what the Xbox 1 did). So if you have the game installed to your HHD already, it will be just like copying the files to a different folder (and on the same drive).