You don’t change to a bigger van & system just because you can afford to.

True.
But not many ppl on here are happy with their business as it stands. The vast majority are trying to improve and expand, to earn more profit in less time.
The bigger van will prob only cost a little more, and the safety aspect gained by not pushing the vehicle to its weight limit (or above on busy days!) will be more than worth the small extra investment. Running costs will be nearly identical, but earning potential will vastly improve.
I don't have a massive business,.. I'm working on my own most days, and I prefer it that way. I have a Pug Boxer van and wouldn't recommend anything smaller, unless you clean windows but do none of the extra's like powerwashing etc.
I've a 400 litre tank, and find it too small. My biggest job (also my biggest per hour earner) takes over 800 litres of water,....
I'm planning on adding a 2nd tank,.. but most days I'll only fill 1 tank.
I had a smaller van, and I earned less with it. It did cost the same in fuel/tax/insurance though! Changing to a bigger van cost me a lot,.. new roof rack, towbar, sign writing, protectacote, getting the tank cage fitted etc etc. If I'd gotten the bigger setup initially I would have saved myself over €1000!!!
So the "smallest system you can get away with = more profit" idea just baffles me,.. I honestly can't understand where you are coming from Ewan!
Do you have a setup yourself? Are you a window cleaner?