The problem is really heat dissipation. Intel could easily make a 15W or 28W quad core CPU, but that would be a bit pointless, because that CPU will most likely not be any faster than a 15W or 28W dual-core CPU. The thermal limit would be just to severe for a multi-core configuration. I am not aware of any manufacturer that uses a quad-core CPU in a chassis comparable to the 13" MBP.
But speaking theoretically, yes, definitively, I'd love for my 15" configuration to be as thin and light as the 13" machine.