Also the smaller the cubic area, the quicker it will get to explosive potential.
It will achieve the Stoichiometric ratio of 15.5:1 quickly, but will also exceed it quickly. Once you get past the optimum concentration it won't explode as there isn't enough oxygen in the mix, but it will burn around the edges. If it does explode, then the smaller volume will do less damage and it will probably be contained by the vehicle structure.
Personally, I wouldn't be keen on opening the boot though. A poor connection to the boot light might be enough to ignite it as all the air rushes in as the lid opens and your face would be right in the fire ball.
Plumbers vans seem to be over represented in gas accidents, if you are lucky its using the central locking, unfortunately more than a few are from opening the side door.