Is there a functional difference between the underbarrel extinguisher (in attachment-vend) vs the mini-extinguisher vs big extinguisher aside from capacity?
Bonus Question: How do I refill them, can I use any water tile or do I need to find water tanks? Do sinks work?
Bonus Question: Is there any convenient way to carry the big extinguisher outside of the drop pouch?
Thanks in advance 
you can fit the big extinguishers into fuel tank pouches. i usually put one napalm tank and one extinguisher in that pouch, a ube and small extinguisher in the webbing when i bring the flamer because i find having the utility of being able to use flames on the front more freely better than extra ammo
I prefer the bigger extinguisher, this way when someone I don’t like is on fire, I can beat them to death with it, and afterwards say, “Why didn’t it extinguish them?”
Also, if you wear the firesuit that is inside of fire lockers, you can wear the extinguisher on your exoslot like it was a weapon. Wow!!
Only refil them with water tanks. I once tried to refill my spec flamer tank with a sink and it broke it so I couldn’t refil from any water source. Might be fixed but trauma is still there.
I just stick a ube on my shotgun since the accuracy doesnt really matter for PBs; its got better capacity than the mini one which is kinda pathetic, and the fullsize one doesnt fit into like 90% of storage slots so is also not really worth taking.
There’s not any big difference, no, but you might find one or the other easier to use due to just having to ready the mini and big once and have it on demand in your storage compared to needing to flick the underbarrel on and off a gun, which could save you some time or lead to a faster extinguish.