OK, so I'm a bit dim. Where does the weight come into it, except you don't actually know what it is? It is the same weight as if the bag was checked.....Do you actually add up the weight of the checked baggage? If you have 200 pax and they each have an extra 10 kilo, that's 2 tons more - but if they all weighed an extra 10 kilo, you wouldn't know.
I got charged excess baggage by American Airlines becasue I was 4 lbs over the limit. In one bag. Coming back, I had more weight in luggage but two bags, so no problem.....
But either you pay in terms of fare on a classical model full fare airline (although they have so many deals that they can be cheaper than a loco at times) or you pay for water, sandwiches, coffee, checked bags, early boarding and so on - not sure about better weather on your holiday but probably! - and it may not prove any cheaper at the end of the day.