There's always going to be winners and losers in the colony rush.
Any race which doesn't have an advantage (like the Thalan's inherent social/military bonus) that allows them to crank colony ships out faster or move colony ships faster than other races is going to get boxed in. However, colony rushers get to pay through the nose for all their colonies which don't produce much tax to begin with.
However the reality is that the rushers are always ahead in the tech race because every colony produces research. As soon as their economy recovers to the point where they can run the production slider at 100%, just that amount is enough to make things very grim for a smaller race.
In theory, a race with just six colonies could get started on Planetary Invasion real quickly and then gobble up bloated empires before they know what's happening.
Now for a human, it's simple to appreciate just how slim this particular window of opportunity is. An early war to grab planets from your upstart neighbour is a great idea, but it can backfire if your enemy researches Planetary Invasion and Space Weapons and starts sending transports and fighters to claim back the worlds you've plundered.
The AI can't appreciate the potential rewards or risks in this strategy, so it usually does not have its pieces in position. A small empire under control of the AI usually falls behind unless a mega event turns things around for them, or the human player intervenes (gifted ships, planets, techs) or they just get lucky and happen to grab some planets during a dogpile war.