The devil is in the detail
I own a late-2011 MBP 15" 2.2 Quad, and am considering moving to a 13" Retina 3.0 Dual.
The overall Geekbench results are ~10,000 for the 15, and ~8000 for the 13, but that's probably because the 15 has twice as many lower clocked cores.
Most of my work is code compilation, and I believe this is a single-threaded integer workload.
Under these workloads, the 13 scores much higher, e.g.
Blowfish single-core scalar
13: 2685, 15:1932. 39% higher on 13.
Text Compress single-core scalar
13: 3102, 15: 2580. 20% higher on 13.
Image Compress single-core scalar
13: 2677 ,15: 2261. 18% higher on 13.
Average: 26% higher on 13.
These data were from a 2.9 13, where the clock speed is 32% higher than the 2.2 15, so it's hardly surprising that the 13 does better.
Even 15" MBPs posting overall Geekbench scores in the mid-12000s are only managing ~2750 in the Blowfish single-core scalar, which is within 3% of the latest high spec 13".
So basically, YMMV.