Ricoh G900 has a 20.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor . On the other hand, Canon SX510 HS has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor and features Digic 4 processor.
Ricoh G900's sensor provides 8MP more than Canon SX510 HS'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 G900 and SX510 HS sensor size comparison.
Ricoh G900 and Canon SX510 HS have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.