Pixel Pitch — The physical size of each pixel on the sensor. Smaller pixels (higher megapixel count for a given sensor size) hit the diffraction limit at wider apertures.
Aperture — Narrower apertures produce larger Airy disks. Past a certain f-number, the Airy disk exceeds the pixel size and detail is lost to diffraction.
Sensor Resolution — Higher megapixel sensors demand more from optics. A 100MP sensor is diffraction-limited at a wider aperture than a 24MP sensor of the same size.
Wavelength of Light — Green light (~550nm) is the standard reference. Shorter wavelengths (blue) diffract less; longer wavelengths (red) diffract more.