Sony A57 has a 16.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor . 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 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 A57 and A65 sensor size comparison.
Sony A57 and Sony A65 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 compared to Sony A65, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.