Its because the springs are shorter mate, the standard bump stops are there to stop the suspension from bottoming out. With a standard spring you should'nt hit the bump stop, but with a shorter lower spring you run the risk of the car sitting on the bump stop instead of the spring. Not good.
When i bought my Koni's they came with new bump stops they were tiny compared to the genuine ones. They aint hard to cut mate a junior hack saw will do, just try and keep the cut level.