Fujifilm X-A2 has a
16.0MP APS-C (23.6 x 15.6 mm ) sized CMOS sensor and features EXR Processor II processor. On the other hand, Nikon D40X has a
10.0MP APS-C (23.7 x 15.6 mm ) sized CCD sensor and features Expeed processor.
Fujifilm X-A2's sensor provides 6MP more than Nikon D40X's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the X-A2 and D40X sensor size comparison.
Fujifilm X-A2 and Nikon D40X have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.