Canon A2500 has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Canon SD780 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the A2500 and SD780 IS sensor size comparison.
Canon A2500 and Canon SD780 IS have the same 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 Canon SD780 IS has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Canon A2500, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.