If you have weak springs, then some of the available weight movement will be used up at idle leaving less at full advance because the weights would be at their stops at a lower RPM. Think of where the advance would be if you had no springs. The weights would be fully extended at idle, but you would adjust the plate to move the timing to the idle timing mark and there would be no more available movement of the weights possible so you would not advance past the idle timing mark. Having no springs is the extreme example of weak springs that results in less available full advance.
If you had very stiff springs, like a wire in the place of the springs, then idle timing would be attainable, but there would be no movement available for advance timing. This is an extreme example of stiff springs. What actually happens with stiff springs is that full advance is delayed to a higher RPM, perhaps even higher than your normal cruise RPMs so you do not get any advance for normal riding.
So, either loose springs or tight springs will result in a decrease or delay in full advance at whatever RPM you would normally expect to see full advance.
