Right, but the problem is trying to fit 5 gallons of beer in a 5 gallon bucket. There isn't any headspace for the krausen during primary fermentation and yeast will be flying out of your airlock or blowoff. You can buy two and spilt a 5 gallon batch between the two or brew up as much as 8 gallons, splitting around 4 gallons per 5 gallon bucket.
"Good people drink good beer." -HST