This could be due to either the angular dependence of image pixels or the design (level of offsetting) in the microlenses.
If you imagine an image pixel as a bucket with the light sensitive part in the bottom, light coming from directly above will hit the bottom with no problems. However the more oblique the angle of light the less will hit the bottom as it is blocked by the sides. This is sort of how a pixel works in a front side illuminated sensor, there are wires and other structures in front of the actual photosensitive area.
If you have a higher resolution sensor then the pixels are smaller but have the same "depth", so if you imagine a bucket with the same height but smaller diameter, the angle over which light can hit the bottom is greatly reduced.
In order to try and mitigate this issue, digital camera sensors have microlenses which try to focus the incoming light onto the small photosenstive area within each pixel. Toward the edges of the sensor (where incoming light angles are naturally shallower, and where vignetting occurs) offset microlenses are sometimes employed to redirect light so that it enters the pixel at a steeper angle.