Both Nikon W300 and Pentax WG-2 have 16.0 MP resolution sensors but Nikon W300's sensor is 1/2.3 (6.17 x 4.55 mm ) and Pentax WG-2's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Pentax WG-2's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon W300.
Below you can see the W300 and WG-2 sensor size comparison.
Nikon W300 and Pentax WG-2 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 Pentax WG-2 has 15% larger pixel area (2.04µm2 vs 1.76µm2) compared to Nikon W300, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.