Canon 1D X has a
18.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual Digic 5+ processor. On the other hand, Canon SX40 HS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor .
Canon 1D X's sensor provides 6MP more than Canon SX40 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 1D X and SX40 HS sensor size comparison.
As seen above, Canon 1D X has a 30.8x Larger sensor area than Canon SX40 HS. 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.