Let's round that up to a full three bytes. We've lopped off one byte already. The RA for a given zone does run through a full 360 degrees, also stored in the .01 arcsecond units. Therefore, the RA can be stored in sixteen bits, a.k.a. two bytes. We're down to nine bytes a star.
Further compression does get a little harder to do.
The declination values will vary almost randomly across those 7.5 degrees within a zone, and by definition, you can't compress random data. With a half billion stars in A2.0, we'd be talking about saving 125 MBytes. Abandon that restriction, and all sorts of wonderful possibilities open up. In fact, Dave Monet might well insist on it from a legal perspective. But A2.0 consumes about 6.3 GBytes in its 'raw' form.