I used to hunt with a guy who was a brew master. He worked at Budweiser before turning craft brewer. He said they would process the beer at high concentration to save valuable processing space and then in the end water it down before filling it according to product: Bud would get some water, and Bud light would have a big water pipe come in. Ugh!
Honestly, I don't get light beer. I'd rather have a single good beer and drink water after. Or, again, just stay with a decent N/A. Strangely you never drink a sixpack of those in one sitting!
G