This has been ongoing for a long time and has been unaddressed. The trailer update PARTIALLY fixed the type 5 engines, by giving them a capacity of 10, but the lists in equipment descriptions doesn't match up because the items require more than 10 capacity.
I got tired of asking them to fix it.
Well...I figured while it's not seriously game-breaking, at least there's a neat table to show them exactly what needs fixing in the future. As long as it gets into their bug tracker.
Upon closer inspection, it appears there are multiple vehicle capacity & equipment size mismatches currently showing in game for fire vehicles. I don't know if this is due to the new trailer update that dropped a few weeks ago or if this has been going on longer.
See the table below for complete details.
Also, at one point, the Type 3 engine had capacity (although I don't remember if it was 10 or 20). I can think of at least three departments in our county alone that use combination Type 1/3 engines, so having that capacity back would be nice for more realism.
If you need screenshots for proof, I can provide those.