Forgive me if this is obvious from the XPH text, right now I've only got d20srd.org available to check.
Q10: When a Large creature manifests augmented expansion to become Gargantuan, does it suffer the -2 size penalty to attacks and AC mentioned in the power, or the -3 mentioned in the monster manual for a Large > Gargantuan resize?
It is not entirely clear if the designer had been wording the power with the impression that the user would be a medium sized PC or not. The 1st paragraph suggests that this might be the case, yet the 2nd paragraph mentions something along the line of "...if your new size is large or larger...", which at first glance seems to suggest that you still get only a -1 size penalty to AC, even in the case of say, a gargantuan dragon psychic warrior1 manifesting expansion to make itself colossal.
Common sense (for me at least) would suggest that you simply use the size penalties to AC as listed in the MM.
It simply seems like a case of very sloppy copy-and-pasting the text for the enlarge person spell from the PHB. This would not have been problematic for enlarge spell because it can only be cast on humanoids, and to this day, there are no large humanoids. However, they probably neglected to take into consideration the fact that expansion could in fact be manifested by non-humanoids (who in turn, can be of any size), and thus did not revise the power entry appropriately.
At least, that is my take on it.