Olympus TG-5 has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features TruePic VIII processor. On the other hand, Olympus TG-870 has a 16.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features TruePic VII processor.
Below you can see the TG-5 and TG-870 sensor size comparison.
Olympus TG-5 and Olympus TG-870 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 Olympus TG-5 has 32% larger pixel area compared to Olympus TG-870, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.