Honestly I don't know if I think this is ethical or not, but I do know that anything justified with "the other guy is trying to screw me over so that makes it ok" generally doesn't fly with me (no pun intended).

Airlines are squishing more seats on the plane and charging for anything that's not nailed down. And peanut butter manufacturers are making bigger indentations at the bottom of the jar, so the same "size" jar holds an ounce less product, but the price doesn't go down. Doesn't mean it's ok to go to the Publix and scoop a spoonful from a second jar. And the fact that Peter Pan was trying to get 10% more profit from you doesn't change that.