Sony a5100 has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor. On the other hand, Fujifilm X-A1 has a
16.0MP APS-C (23.6 x 15.6 mm ) sized CMOS sensor and features EXR Processor II processor.
Sony a5100's sensor provides 8MP more than Fujifilm X-A1'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 a5100 and X-A1 sensor size comparison.
Sony a5100 and Fujifilm X-A1 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.