Hi, regarding to the first issue this could be helpful: RealityCapture XMP Camera Math. Or you can try align undistorted images inside RealityCapture and then export wanted parameters.
Regarding your second question, that is expected, as Fit will provide new image according to a best fit. To have the image in same size as original, you need to use Resolution: Preserve.
- Fit - keeps the resolution of the image created in the previous step.
- Preserve - The resolution of the undistorted image is the same as the resolution of the original image.