The current status of the leaks clearly indicates that there will be no new main sensor in the iPhone 15 pro/ Ultra and that the IMX803 will remain. However, the IMX803 was an enormous leap compared to it's predecessor in terms of sensor size. At 1/1.28" it is almost as big as the 41MP sensor of the Nokia 808 pure view was (1/1.2").
The size of the IMX903 is currently speculated at 1/1.14" and thus would be a significantly smaller jump purely in terms of sensor size than between the IMX703 (iPhone 13 pro/ iPhone 14) and the IMX 803 (iPhone 14 pro/ iPhone 15 pro).
Also, the current speculation is that the "normal" iPhone 15 will get a 48MP sensor, but it's smaller than the IMX803 of the iPhone 14 / 15 pro / pro max / Ultra.
What must also be taken into account is that larger sensors with a specific focal length build deeper (the lens must be farer appart from the sensor), which is why the focal length of 24 mm for the IPhone 14 pro/ pro max is already very small (iPhone 13 pro was 26 mm). Historically, over 30mm (iPhone 7: 35mm) was more common for the standart main camera. Even shorter focal lengths based in larger sensors increasingly turn the main sensors into ultra-wide sensors with all the disadvantages, or the camera hump becomes more and more extreme. This is also the background of telescopic sensors.