Canon 1200D has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Canon T7 has a 24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4+ processor.
Canon T7's sensor provides 6MP more than Canon 1200D'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 1200D and T7 sensor size comparison.
Canon 1200D and Canon T7 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 Canon 1200D has 33% larger pixel area compared to Canon T7, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Canon 1200D and Canon T7 sensors have been tested by DxO and the results show that T7 has a better overall score of 71, 8 points higher compared to 1200D's score of 63.