Sony A100 has a 10.0MP APS-C (23.6 x 15.8 mm ) sized CCD sensor . On the other hand, Sony HX400V has a 20.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Bionz X processor.
Sony HX400V's sensor provides 10MP more than Sony A100'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 A100 and HX400V sensor size comparison.
As seen above, Sony A100 has a 13.3x Larger sensor area than Sony HX400V. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.