It's more nuanced than the question asks. In terms of power, it is basically as simple as 'its windier up there' as there can be as much as 2-5 mph difference 25 feet above the water surface. Placing more sail area up top takes advantage of more power.

Now we get into nuance...Square top sails are usually designed to match the mast, which has a very dynamic interaction with the sail plan. The front of a performance cat sail is cut with a curve in the luff (the part that goes into the mast track) which, when the mast bends, allows the sail to flatten. Without going into a lot of detail (you can spend hours trying and still not be able to visualize the process until seeing it with your own eyes on a boat) the square top works in tandem with the cunningham/downhaul/rotator to impose a shape on the sail that is most conducive to sailing conditions. The square-top provides leverage on the top of the sail down from the mainsheet (think square top 'pulls more downward' vs than does a pin-head sail (think 'pulls more backward'), and is therefore able to bend the top of the mast more efficiently. The result is greater ability to tune the rig according to sailing conditions, and an edge on speed overall.

That said, I haven't figured it all out...

Hope that helps