why the image or output in HARMONY changed , when my image is rendered in low resolution ( 720x405 ) and rendered in hi resolution (1920x1080) using a matte resize module.
You have posted under the wrong forum but you need to provide details.
Changed how exactly? Why are you using a matte resize module?
There is a Scale Output module made specifically for doing just that - scaling the output. You can find it in the “Filter” section of the module library.