Canon 600D has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Sony A65 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz processor.
Sony A65's sensor provides 6MP more than Canon 600D'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 600D and A65 sensor size comparison.
As seen above, Sony A65 has a 1.1x Larger sensor area than Canon 600D. 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Canon 600D and Sony A65 sensors have been tested by DxO and the results show that A65 has a better overall score of 74, 9 points higher compared to 600D's score of 65.