The reaction chamber gives time for organic material to attach to the bubbles. A larger reaction chamber isn't a horrible thing, if the pump is sized to it. This is basically what dictates what size tank a skimmer is rated for.
Most skimmers( which are in sump models) Discharge water in the same compartment as they pick it up. Basically, as long as the skimmer is submerged, it's cleaning water, and it will only remove what it's capable of, no matter how much new or old water it's exposed to.
As far as manufacturers cutting costs, do you think a foot of pvc
is going to make a noticeable difference if they can boast that their skimmer is clearly more efficient than the next guys? Doubtful.