If you want to apply the same deformation but on a smaller image, then you do not need to change the homography.
If you want to change the scale factor of the homography, you can obtain the new homography matrix by multiplying the previous one on the right by: [ 1/lambda 0 0; 0 1/lambda 0; 0 0 1 ]
(in row-major order), where lambda
is the desired scale factor.