CS has a minimum take off weight defined by the deceleration rate in case of engine failure during capture mode.
Took a while to get my head round it but it's summarised thus:
Light weight gives very high climb rate and therefore it will enter ALT CAP very early. If an engine fails at that point the capture manoeuvre is not modified and so the aircraft starts bleeding speed trying to fly the curve. This can put you into underspeed pretty quickly if not addressed. Transport Canada didn't like it. So the fix was to restrict the minimum take off weight and therefore thrust to weigh ratio.
Lower climb rate means later ALT CAP and less time in the capture manoeuvre. Also slower deceleration after the failure which gives the crew more time to react.
A future modification to the AFCS software we are told will remove the restriction.