Both Nikon D3400 and Nikon D7200 have APS-C sized 24.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another similarity between these two cameras is that both Nikon D7200 and Nikon D3400 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the D3400 and D7200 sensor size comparison.
Nikon D3400 and Nikon D7200 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.