Also add in the common areas, lounge etc, galley where the food is prepared for everyone, lockers for that food (and on a passenger ship they won't be eating tinned survival rations

), crockery and cutlery draws.
You can laugh at the crockery but make a pile of enough plates, bowls, cups, wine glasses, beer mugs, saucers, knives/forks/spoons, cooking and food prep items, water boiler, wine/beer chiller, fridge, freezer etc for 10+ people in the average small merchant. Add to that a small galley area to handle the food prep and lockers to hold enough reasonable food for 10 people for a month.
The barracks may allocate 2dtons per person but like a stateroom a lot of that goes on stuff other than the sleeping area. Once you add the galley/dinning area/head/lifesupport in you probably end up with 2 or 3 bunks in a single dton on a starship.
Waste disposal as Locarno24 mentions is going to be a lot more complex than somewhere to dump the food scraps. Is the water filtered and purified for recycling. Solid organic waste probably gets packed and either stored for disposal at the next starport of dumped into space towards the closest sun. New encounter, hit by frozen block of someone elses waste dump
All in all there are a lot of uses for the 4dtons of a stateroom that have nothing to do with the actual cabin.
One point to consider is that a dton is 1.5 x 3 metres by 3metre high. Dropping the height to 2.75 metre will add approx 10% to the useable floor space while still allowing 30cm (1 foot) of cable/pipe runs above the false ceiling and room for the grave plates below the deck with 7 feet of passenger head room. Simple holo false ceilings can make them seem a lot higher for the passengers comfort, just no wookies on board.
10% isn't much but when you are playing with 10 staterooms on a small ship that adds another 4dtons of space on the floor plan. Fits into the wiggle factor just don't do it for the cargo bay or those standard cargo pallets don't fit any more
