They did do you a favor, your lights are brighter. BUT that probably
doesn't have anything to do with the burn-out.

Check a couple of things;
1. What is the rated life of your bulbs?
2. How long are you running them? Timer or photocell or both? If your
running them all night and your neighbor isn't your going to lose bulbs
(and money) faster.
3. As below, check your incoming voltage. Are you getting 115-120V?
4. Check your out going voltage from the transformer. Many transformers
have multiple taps to help overcome line losses. Check it at the
closest fixture. If its over 11.5V you may want to drop it down.
5. Do you like having brighter lights? If so you may want to just live
with it.
6!! Check for voltage spikes! This is probably your problem. A motor
starting up or some other electrical problem (loose neutral, large
factory near by) can give you a spike that will burn bulbs in a well
designed system. This is the number one cause of premature failure.
7. Check for overheating in the fixtures. Not usually a problem
outdoors but killer for lamps.

Putting a dimmer in *will* get you longer life.
1. Get a transformer rated dimmer!!!! You could lose both the dimmer
and the transformer and that will cost far more than the extra bulbs.
2. Put it in the 120V side.

RickR