It would be great to find out what causes this. Pfeffersack has suggested it's because he starts more isolated than the other factions, and thus has room to expand, but in that case the Spartans should perform equally well. In my game they were indeed powerful, but still way behind Yang.
The isolated start is one thing contributing, but it goes on if you have a close look at all the other advantages Yang gets. Nearly all of the seem to be prefctly suited for the AI:
* The AI isn't too smart in combat; it relies on sheer numbers often - +10% production and double speed Infantry help them greatly
* the AI is probably not to good at practising "mobile defense", too...so they will be caught in their bases often...and here those 50% help (they also help vs. NL, right?)
* no pillaging is useful as well - the AI is less smart and efficient in rebuilding, so any damage not dealt here is for the good
* Drones eating less food leads to higher population, for whatever it is good. Surely, the human player can send Yang's bases easily into disorder - but will the rest of the AIs be so intelligent as well?
The Spartans boni on the other hand are probably not so powerful in the hands of the AI:
* Cloned Units with full special ability slots are nice - but does the AI exploit that really? In the way they will on purpose focus on grow to "draft away" the excess population?
* MP might not come into play on the higher levels, where the AI gets happiness boni anyway (also unhappiness isn't theat problematic in PF at all)
* GGs are another feature that the AI uses probably suboptiomal (several choices what to do, likely they don't always pick the best and at least 1 of them is extremly problematic for the AI [promoting units])
* The Commando promotion shines best if you know how to use your units; the way the AI uses them it is just a minor strength upgrade
(* Starting outside the PC might be a disadvantage, too - but honestly I haven't noticed anything proofing this theory so far)
Also most of the Spartans boni seem to come into play/begin to shine later...and the beginning matters most, as we all now. Their might be also character differences, which fuel this.