W/G Calculation...

[greenmaji]

the ballasts for CF fixtures have 128 watts total.. for the 55's and 65's.. these ballasts are used in NOapplications (4-F32T8 ballasts, they use two leads per bulb) as well (greater number of bulbs) only a few manfactures have specialy made ballasts that will only handle a certian wattage of bulb and corlife I think is one of them.. (I would have to check them out)
HTH
Fulham is the one of the most used manifacures of ballasts.. check out there recomendation they dont even bother listing the 65's..
http://www.fulham.com/afbal.php

for a dual 55/65 watt fixture a manifacture would use this fulham ballast..
http://www.fulham.com/Detail_Ballasts.php?ID=WH5-120-L

too much time DIY reserching..

 

[Simpte]

The 55/65 watt bulbs produce almost identical lumens. The main difference is the 65 watt is about 1 inch longer. It sounds like you have the same fixture as I do (Odyssea 4x65). I was not happy with the bulbs. I switched to 2 9325 GE bulbs from atlantalights.com and run those with 2 6700K bulbs. Much happier with the color.

 

[malkore]

I've never seen a difference in size on 55 vs 65 bulbs. In fact, I've never seen them advertised/sold anywhere as anything but 55/65w bulbs...implying it's not the bulb that's different...its the ballast driving the bulb.

65w is more common on non-US manufactured fixtures because other countries are a little more lenient on safety. Bulbs driven to 65w run hotter, and burn out sooner...its almost like overdriving a 55w by 10watts...not a lot, but it does make a difference.