Nikon D3400 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Expeed 4 processor. On the other hand, Kodak Astro AZ651 has a 21.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor .
Another difference between these two cameras is that Nikon D3400's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the D3400 and Astro AZ651 sensor size comparison.
As seen above, Nikon D3400 has a 13.1x Larger sensor area than Kodak Astro AZ651. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.