Sony A65 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz processor. On the other hand, Sony A57 has a 16.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor .
Sony A65's sensor provides 8MP more than Sony A57'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 A65 and A57 sensor size comparison.
Sony A65 and Sony A57 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Sony A57 has 49% larger pixel area (22.87µm2 vs 15.28µm2) compared to Sony A65, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.
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. Sony A65 and Sony A57 sensors have been tested by DxO and the results show that A57 has a better overall score of 75, 1 points higher compared to A65's score of 74.