Itīs nothing to do with how many Liters, really.
Without ADR you are allowed to transport Dg with a maximum of 999 Points.
Jet A1 1 point per Liter - AvGas 3 points per Liter. Thatīs why Jet 999 Liters allowed - AvGas 333 Liters allowed.
And in Europe you can use a bowser that has more capacity as long as you are certified to drive the car-trailer / truck combination with your driving license.