Canon 70D has a 20.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor and features Digic 5+ processor. On the other hand, Canon 80D has a 24.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor and features DIGIC 6 processor.
Below you can see the 70D and 80D sensor size comparison.
Canon 70D and Canon 80D 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 Canon 70D has 20% larger pixel area compared to Canon 80D, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.