When Leica introduced its first digital rangefinder in its venerable history, it came with an APS-C sized sensor. At the time, Leica (and others) said that a full-frame sensor in a rangefinder would not provide reasonable quality with wider lenses as the sensor's distance from the lens was too close and there would be unavoidable vignetting.
Two years and one model later, Leica has released a full-frame 18MP variant of its M-mount rangefinder. Presumably having worked out the vignetting with advances in microlenses or simply magic, the M9 provides the same exceptional sharpness of the M8 at 2fps. Besides the increased resolution, the M8 also has improved sensitivity in low-light, supporting up to ISO 2500.