that's a though one, IRA.
a good example of volume concerns is a cookpot. it's hollow, and therefore a waste of valuable space. but, if i can nest things inside the pot then the space is not wasted. now lets say that i have a stove that is very efficient and fits in the pot and leaves room to fill the pot with other things, like fuel, utensils, or just stuff. if that stove weighs 4 ounces or 1 ounce will make absolutely no difference to me. 3 ounces one way or the other just doesn't matter in a kayak. hell, three pounds
one way or the other won't make a difference in a kayak. it sure would in a backpack though!
lets take this to the extreme.
lets say the kayak has a maximum weight limit of 350 pounds. if you read that from a kayak manufacturer, it includes the weight of the boat. so 50# for the boat leaves 300# for paddler and gear. me and the gear i wear weigh 190#, leaving 110# for stowable gear. a full can of beer weighs 13.1 ounces. 12 dozen beers (loose) would easily fit in the boat, and over-load the boat's capacity by about 9 beers. at this point the boat would be quite tippy and hard to handle. after all those beers the paddler would be quite tippy and hard to handle.
an over loaded kayak will be hard to handle.
an under loaded kayak can be hard to handle as well.
one of my kayaks absolutely loves to have about 50# of gear in her. she really settles down and is a joy to paddle.