Info and data as to why running the same boost pressure from a small turbo doesn't get you the same power as running from a big turbo:
http://www.motoiq.com/MagazineArticl...s--Part-2.aspx

 

Good stuff. There is something that is also worth pointing out. Provided you have a good setup with minimal restriction, peak torque usually doesn't change a whole lot with different turbochargers.

The exception to this I have noted though, large turbos (GT42+ on a 2.0L) operate well into crossover conditions at midrange (5500-7500 RPM) engine speeds and moderate boost levels (20-35psi). This is also where most typical cams want to peak out on torque, along with well designed manifolds and typical head work. All this can combine pretty powerfully and I've seen 15-20% VE gains over GT35R sized setups...this has also been accompanied by poor BSFC since a bunch of unburnt fuel is going out of the exhaust during overlap by the higher intake manifold pressure than exhaust manifold pressure.