@Jace:The size of the fixture alone is responsible for the temperature of the air inside, what may be 40degC for bigger or 80degC for the small fixture. This 40degC look like huge difference, but it remain as the same absolute temperature difference for all components inside. So for reliability of the ballast and/or ignitor it is significant difference, you have to remember, then the arctube run way above 1000degC, while the 40K difference remain the same even here - and 1000 vs 1040degC is nearly not any factor compare to other things, what influence the arctube temperature. By the way this fact is the reason, why HID's seems to be insensitive to the outside temperature - the outside temperature simply does not vary in the view of the arctube...
Way more important is the shape of all heat reflective surfaces - how much of the heat radiation they redirect back onto the arctube - these may shift the arctube temperature easily by few 100's degC and this then has large impact on the lamp characteristic.
Very strong factor are ballast load characteristics and tolerance and it's response to mains variation.
And last, but not least all factors, that affect lamp aging: Switching frequency, burning time per day, current crest factor, lamp itself,...