All this idea will do is shift the problem from the smaller craft seeming (unrealistically cramped) to the larger vessels looking stupidly spacious it solves nothing.
Also in RP terms why would you transport people in a Rhino or any other cargo vessel for that matter. The issue with numbers of people vs size of model might only really be an issue for the Democritus and the Pelican, two might i add rarely seen vessels. Liners are there to do that job and its currently realistic the number of people they can carry.
Another RP point, if you increased the unit per passenger its basically saying we have increased your ticket price by making your room the size of a tennis court, great for the super rich (hence VIPs currently have larger rooms) not great for the average passenger.
I would suggest simple 1 unit for passengers, 2 units for vacationers and 4 - 5 for VIPs
