Its hardly a surprise that this worked. I tend to do the same thing from time, especially if I am near a civ at the start which is likely to be hostile to me.
The AI pretty much has to be generalized, it does not alter its initial buildup methods too much with respect to planet availability or galaxy size. (I concede I could be very much wrong here, its not like I wrote any of this AI). The AI starts out trying to colonize as many nearby planets as it can. If you take out its home world with a rushed invasion, you can pretty much take them right out of the game at the start.
If you try this on a larger galaxy though, you will probably only manage to take out one of your neighbors. While your screaming along towards invasion tech, the other AI's are going for diplomacy and trade, and will end up trading tech among themselves as they always do. Assuming you manage to stay out of any wars after your initial blood soaked expansion, you are in a pretty powerful position, however.
If the AI put more effort into engines / sensors, I do not think this would be a viable approach. All it would take to foil this sort of invasion is a minor military starbase, and maybe 2 fighters with impulse engines. If your initial strike is repelled, your boned.
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