Headlight bulbs can get dimmer with age - especially the high powered ones. The extra heat can discolour the glass over time leading to reduced light output.
More likely - the extra current drain from the higher powered bulbs causes a higher voltage drop in the cable feeding them - double the wattage and double the voltage drop. Its pretty typical to lose up to 2-3 volts in the wiring to the bulbs with normal wattage bulbs - put a 100 watt bulb in and you're almost down to half voltage with some wiring.
I run Osram Silverstar bulbs in the missus fiesta (55/60 watts) and they are much brighter than the standard 55/60 watt bulbs in the cavalier. This is due to the increased xenon gas in the bulb causing the bulbs to glow a lot brighter.