For a long time there were no large or even medium size compacts but now they are starting to appear in numbers, with cameras like the Sigma DP1, Fuji X100 leading the way. Most of these cameras are on the large side and feature prime lenses.

There are a number of interchangeable lens compacts with a variety of sensor sizes from the very small pentax Q (5x crop), Nikon 1 (2.8x crop), micro 43rds (2x crop) and Sony NEX, the largest of the lot (1.5x crop).

Recently Canon announced the G1 X, with a m43ish size sensor and more traditional compact features and handling.

Sadly most customers of compacts aren't concerned with lowlight image quality (daylight image quality is good, even with a small sensor, and deep DOF makes these cameras easier to use)

There's no technical reason for the relative scarcity of large sensor compacts, after all one time most compacts used 35mm film, so a full frame pocket camera is possible. The main difference is that 35mm compacts tended to have prime lenses, and the ones with zooms were very slow (f/5.6-f/8).

Nowadays people are more than willing to trade sensor size for zoom range. You simply can't make a fast 20x zoom for a large or medium sensor and get the thing in your pocket!