I have a 640x480 composition, in which I am playing 320x240 clips.
In order to make the clips fill the composition, I change the scale in the layer properties (or in the clip properties, result is same) to 200%. This works fine.
However, when I disable any of the color channels for the clip (RGB), the clip shrinks back to 320x240, in the lower left hand corner of the composition. Re-enabling all the color channels restores everything to normal.
This happens regardless of whether I apply the scaling at the clip or at the layer level.
640x480 resolution clips work fine.