Canon R7 has a
33.0MP APS-C (22.2 x 14.8 mm ) sized CMOS sensor and features Digic X processor. On the other hand, Canon 400D has a
10.0MP APS-C (22.2 x 14.8 mm ) sized CMOS sensor .
Canon R7's sensor provides 23MP more than Canon 400D's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the R7 and 400D sensor size comparison.
Canon R7 and Canon 400D 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 Canon 400D has 220% larger pixel area (32.60µm2 vs 10.18µm2) compared to Canon R7, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.