If this were an issue, commercial breweries wouldn't be using 12 ouncers at all would they?
You're over thinking/worry about this, besides the headspace is really the same regardless of the size of the bottle, you'd just as easily run the risk of oxydizing the beer regardless of the size...if this were something to really worry about.
People often bottle in larger bottles to cut down on the amount of bottles, not for anything like this.