I have not been to Gib, but I understand it is almost a vertical rock face so the population is constrained into a much higher density than 2.6 sq miles would indicate..and of course a lump of the flat land is an airport so constraining it even more. IoM is an open enough island, with many small houses, not tall apartment blocks. So getting out into the countryside is no big deal. Being an Island has advantages also.
Finally there is the statistics of small numbers... 65 deaths and 25 deaths is less significant than might appear. Small number of discrete events are under Poisson statistics and the standard deviation is sq root N
Suspect you are right. Available land for use is only 6.8 km2... And with a population of 30,000 it does have 5th highest population density on earth.
I love the place actually. Most folk don't like it but it has a reality and vibrance to it. Plus it's near some fantastic windsurf beaches, has loads of wind, good food and fairly cheap prices. Those apes are a pain tho... Not that friendly and they, ll nick anything or is that on Spain.
We were once in a traffic jam queuing to get on to the Rock and somebody knocked on window and asked for cash in exchange for required ticket to get on place. I told him I, d buy one at kiosk, we might turn around and go. He sold about 50 useless raffle tickets to other more trusting tourists. Caused plenty of arguments at the kiosk..
Folk handing in raffle tickets and waiting for barrier to be lifted.... Made us all smile.. That's the EU for you. Very enterprising.
If ever you do go, do the tunnels exploration tour. Incredible. The place is a labyrinth.
And stay at The Rock Hotel for a couple of nights. Like going back to 50s.