Fermenting smaller batches in 5 or 6.5G carboys

Is there any reason I can't brew a 3 gallon batch and ferment it in a 5 gallon carboy? Or even a 6.5G carboy?

THe only reason i can think of is that there may be a higher chance of oxidation.

There won't be a higher chance of oxidation. There will be a nice layer of co2 above the wort to protect it.

Initially, that was my leading concern. However, the surface area of the wort exposed to air is the same, whether you put 2, 3, 4, or 5 gallons in a 6.5G carboy. So I don't see how the risk of oxidation is any greater than a full batch.

I suppose one could argue that a smaller batch will take longer to produce enough CO2 to cover the surface area of the wort, but how much longer would that be? A few hours at most?

Nope, nothing wrong with it. I do it all the time.

5 gallon is about the biggest I would go, and have gone, with 2.5 gallon batches. Though I really try normally to have no more than a "gallon" worth of headspace in my fermenters. You can go with 2.5 in a 5 if you have no other options. I wouldn't use a 6 or a 6.5 though.

Revvy - What is the reasoning behind that guidance?