I been thinking about this all day. Jim is probably right on the bad connection. Some of those outdoor fixtures get moisture and corrosion on the contacts and wire connections which means time to replace it as Jim said. Use a volt meter at all places starting with the light fixture. You should have 120 volts + or - depending upon your local power company. They could have just 115. I wonder if there is a perception issue here. Going from a 60 to 75 to a 100 watt bulb in a dark area may not demonstrate much of a difference. Some light fixtures have glass that seem to block more light than they let out. Remove the cover if it has one to compare. Get a trouble light, plug it in inside somewhere and hopefully on a different circuit to see if you notice much difference. Get an electrician if you have trouble with any of this. Post back with what you find out.