Different kites have different power and range. The trimming of the lines also affect the power. Most beginners once started going will tend to want bigger kites, most will ride smaller when more experience, especially for surf. If you can ride comfortably with smaller kites, it is very rewarding.
Exactly - short and precise answer
Different kites has very different power, yes.
And good words: "If you can ride comfortably with smaller kites, it is very rewarding"
This goes, as you write, for both surf but also freestyle - beginners ride too big kites to compensate for the lack of skill (naturally, nothing wrong with that).
But experienced riders use smaller kites, and jump just as high but with way better "agility" and moves because of the correct kitesize
Weight and windstrength balance can not be compared (measured) really, so will not comment on that one.
Paul - you might just have gotten better now, and found out you dont need the 12 anymore in given conditions, like you used to
So take the 9, and use the 12 for lighter wind.