Master of Orion is one of my favorite games of all time... I wasted countless hours playing it back in the day. And Rich (Ripcord O'Reilly on the forums) is a huge SC2 fan. When we originally started thinking about "Infinite Space" it's no surprise that it was going to be the biggest and most complex space 4X game ever made.
We had all those features like hundred+ star galaxies with multiple planets per star, multiple interconnected tech trees, ten races with ten ship types each ranging from fighters to two-mile-long super dreadnoughts and so on. Our planets even had several layers of different minerals you'd get access to with more advanced mining technology (core-sucking, anyone?), all kinds of stuff we all love to dream about. There's a couple of really interesting screenshots out on the net somewhere
Now, why didn't we finish this game? Several reasons. Mainly, games like MOO3 and imperium galactica 2(?) were coming out and there's no way two or three people could compete with those guys and their megabucks. Secondly the game was becoming so complex that it'd be impossible (at least for me) to write a competent AI, so it turned into a multiplayer-only title and there just aren't enough people playing these games to get together ten of them, especially given that a game might last several weeks.
So basically, it would have taken several years to complete the project and there was no way to tell if anybody would even notice it by the time it was finished, so we abandoned it. A couple of years later SAIS was born out of the work we did on "IS" using a lot of the art assets and background info etc. This time we actually did some planning and designed the game based on what we could realistically do, so it was finished before we got too tired of working on it.
It's interesting to note though, that almost all of the races, weapons and ship systems from the "big game" live on in Weird Worlds, in one way or another.