Sony RX10 II has a
20.0MP 1″ (13.2 x 8.8 mm ) sized Stacked BSI-CMOS sensor and features Bionz X processor. On the other hand, Olympus E-400 has a
10.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor .
Sony RX10 II's sensor provides 10MP more than Olympus E-400'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 RX10 II and E-400 sensor size comparison.
As seen above, Olympus E-400 has a 1.9x Larger sensor area than Sony RX10 II. 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.