The point is the value of the "w" parameter. Yours is set to 210, this means that the width (length) of the slider is 210 pixels, far too much for the width of the main image.
Try to reduce this value (start ie with 50, then increase the value) til you get the wanted result.
You should also remove the w and h parameters in the group line
<group id="player.seeker" x="1" y="20" />
The "w" parameter should be the same for the same parts in dark and light cases, ie left side dark equal to left side light, center dark equal to center light, etc
The width param of the grid, progressgrid and slider should be the same for these three objects.
If the main window cannot be resized, remove the relatw="1" code.