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Thanks, this explains the different sizes of the add-ons, as I have them all saved. The original DR1 5 million star add on was 400 MB, while the DR2 5 million star add on was less than 200 MB.
This is not possible. Let me clear some possible misunderstandings (If there are any):
Gaia DR1 (data release number 1) was a catalog of 1.4 billion stars (exactly 1,142,679,769). But the first release lacked enough observations to perform accurate stellar parallaxes by its own (thus distances). Thanks to the "old" Tycho-2 and Hipparcos catalogs we could establish some constraints on the new Gaia DR1 data so for the stars appearing in these catalogs we were able to yield a catalog of parallaxes putting everything into the mix. From the 1.4 billion stars in DR1 we got only 2 million (exactly 2,057,050) in the TGAS (Tycho-Gaia Astrometric Solution), which is the 0.18% of the catalog. Thus, at most Phunnie could have made a 2 million star addon for DR1 (not 5 million).
In Gaia DR2 (the current most recent data release, until December 3) contains 1.7 billion (exactly 1,692,919,135) stars. Now with lots of parallaxes. But Phunnie based its addon not in all of the DR2 parallax measurements but in a secondary catalog created by Bailer-Jones et al which estimated distances using a statistical analysis on the parallaxes. The Bailer-Jones distance estimates include 1.3 billion stars (exactly 1,331,909,727) of the Gaia DR2 (78.7% of the catalog). Phunnie did a wonderful job selecting only the best estimates and that's why the addon is at most of 30 million stars (which is just the 2.25% of what Gaia DR2 yielded in "distance measurements").
Phunnie could have made a larger addon with currently available data since Gaia DR2 is huge! but some artifacts might appear in SpaceEngine (due to large uncertainties and biases for the farthest stars), making the gameplay seem unnatural. And also because SpaceEngine is currently uncapable of handling that many catalogued objects without crashing.
I think you are confusing the first realease of Phunnie's addon for SpaceEngine with other later enhancements he did. These were all based on the real data of Gaia second data release (DR2), not in DR1 or any other. Now we are expecting for December the third data release (DR3), which will contain more stars. But that is probably not going to change the addon since it is limited by SpaceEngine capacity to the point it can't even give you the entire DR2 now. Thus the improvements for SpaceEngine will come in the form of better parallax measurements, and more precise distances (a 20% better estimates in many cases).