And it makes for a fairly incredible population. As in, probably beyond the 1,000,000 worlds times a 5,000,000 population average.
To Baron Spikey,
All things being left as they are, you are correct. When you take in context like the status the Astartes have in the eyes of normal humans, things get interesting... Loyalties may change on one's opinion of the "Emperor's Angels", and what their mission is.