Smaller pixels would not get you more depth of field, but likey less. Smaller pixels show less depth of field because they resolve sharper details in the middle than were previously resolved with larger pixel lower resolution backs. Hence P65+ has narrower DOF at an equal aperture than say a P25+.
DOF is a function of magnification.
For a given pixel count, smaller pixels give you a smaller sensor.
A smaller sensor gives you less magnification.
Less magnification gives you more DOF
It is a mathematical fact.
Low-res cameras have better DOF partly as DOF is also function of Circle Of Confusion (which depends on the lens and the pixel size). ...so you can be more out of focus without the image being any less sharp than your lens/sensor could be in the middle on the axis of the lens in the middle of the plane of sharpest focus.
If you have a 60Mpx sensor, and you only need 20 for a job where DOF is critical you can improve DOF by using a wider lens and cropping down to a smaller format. (less magnification).
The above applies if you use a smaller (part of a) sensor with the same number of (smaller) pixels. If you use a sensor with the same size but more (smaller) pixels, you would calculate the DOF with smaller Circle Of Confusion, (assuming that the pixel size was the limiting factor for res) and you would get less DOF... but you could the crop and get your DOF back.
...of course the 40lp/mm digital specialist lenses we have now would be out-resolved by pixels smaller than about 4-5 microns, so we would need a new set of lenses.
...of course if you use pixels much smaller than about 2 microns, no lens could out resolve them, as the pixel size would be getting too close to the wavelength of light (. 4 to .7 microns).
