Lighter strings are looser and in order to get the attack and volume you are used to with heavier guage strings must be hit harder. Both of these things will cause the strings to fret out. Also the strings do pull the neck up slightly and lighter strings being looser will pull the neck up less. This will decrease string to fret distance and cause buzz. I use .11 guage Elixirs with the action set almost as close as an electric on my Washburn with very little buzz. It's natural to want the instrument to be easier to play, but in the case of string guage, when speaking of acoustic guitars, lighter isn't always better. In fact I would never go below .11s on an acoustic. Guitars, as with most things in this world, are a compromise. With guitars, one of them is a compromise between playability and freedom of string vibration.
Huh, he eluded to a G-string Huh (spoken in my finest Beavis and Butthead impression)