My problem is kinda the opposite. Some of the good races don't expand. Splink appears to be correct about the 'AI Personality' affecting it.
Drengin, Yor and Korath - Expand just fine. They do get a good territory, about 15-20 planets on a huge map.
Human, Drath, Thal and Krynn - Super expanders. They normally end up with a good 1/6 to 1/4 of the map, about 25-45 on a huge map.
Torian and Iconian - Decent/super expanders... they end up with a huge territory, often colonizing worlds in the middle of other race's territories. Normally about 15-45 planets, depending on how close they are to the 'Human/Thal' personality types.
Arcean, Altarian and Korx - Lucky if they get more than 10-15 worlds on a huge map. I've had instances where they only colonize the other planet in their star system and that's it. Plus they never have any credits when I go to trade with them.