Physical size of Central AC unit
 

I just had a 2.5 ton Trane XL15iAC unit installed (my house is about 1,400 square feet) and it is enormous. If it is so efficient why is it so big? I t seems to me that a unit that big would use more electricity. I keep reading about being careful not to oversize the AC. Does this not apply to the actual size of the unit?

I don't know much about ACs, but maybe it's efficient because the coils themselves are bigger, making the unit itself bigger. I would imagine if the condenser coil covers a larger surface area, it will cool off better, making the compressor work less hard. I might be full of it though, I'm no expert, just speculating.

It is as Red said.

The larger coil surface area. Aids in rejecting heat better. So a smaller horse power fan is used. Which draws less wattage.
A lower head pressure means a lower compression ratio, so the compressor draws less wattage.