Canon SX510 HS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Canon SD4500 IS has a
10.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Digic 4 processor.
Below you can see the SX510 HS and SD4500 IS sensor size comparison.
Canon SX510 HS and Canon SD4500 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 SD4500 IS has 59% larger pixel area (2.81µm2 vs 1.76µm2) compared to Canon SX510 HS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.