Everything comes at a price.....

A larger turbo will give better gains, but you will then have to combat turbo lag. The greatest thing about my little turbo is the fact that it spools almost instantly, whereas a larger one would spool slower since it takes more exhaust gas to spin it.

And you don't want to be throwing the biggest turbo you can find on the car. You want it to be at a nice point on the compressor efficiency map so that it has a nice balance of power and lag and it either isn't too big for your engine to swallow the air it flows and spool it, or too small that it just blows hot air at high RPMs.