VSE: transform a bigger video results in a cropped video

I’m working on a 1280x524 size sequence and I imported a full HD video.
I turned on the Image offset option, so the video is not stretched and in its original format even if it’s outside the video canvas.
Now I want to resize the video and moving to show a particular part, but if I apply the transform effect, Blender resizes and move a cropped version of the video, ie. only what I see in the video canvas will be transformed.
How can I fix that?