If you want a smaller Macbook Pro, you've got to put up with a lower specification processor, but that could change with Intel's newly minted Ivy Bridge processor family. Apple doesn't offer quad-core processors in its smaller 13-inch notebooks due to a hard 35W limit on the thermal design power (TDP). If you want quad-core, you've got to jump up to a 15-inch or 17-inch Macbook Pro, but that could change with Intel's next generation of processors, codenamed Ivy Bridge.
Anandtech reports that Ivy Bridge quad-cores drop the power requirements down to that 35W ceiling, meaning that Apple (and other manufacturers, of course) could more easily offer more power with less... erm... power. Somehow, Intel keeps on dropping the power performance of its processors while adding to the computational power of them. I'm still not entirely sure that magic isn't involved.[Anandtech via MacRumors]