I now tried it with the same jpg:
a black square, 720x576, saved as pict1.jpg
then scaled it to 500x400 in PS, made it grey, pict2.jpg
and scaled 220x176, pict3.jpg
Import in FCP, put them together, the 720x576 is wider then the other 2 next to eachother, height is alright. So it cannot be something with the images itself, like they differ or something.
Does anyone know what I do wrong ? Is anything 720 wide stretched maybe automatically ?