It’s as much the aerodynamics as anything, a 1.6 viavro/traffic is going to burn more diesel than a 1.6 Clio whther they have the same engine or not.
Put it another way, my (1.6 petrol) focus costs £195 a year to tax and does the same/more mpg as a T5 sized van, so that’s the best you’d be aiming for.
talking about aerodynamics and you are not comparing apples to apples, innit?
citroen berlingo van 2014 1.6hdi 75bhp VED £225
citroen berlingo car 2008-onwards 1.6hdi VT 73bhp VED £130
(not perfect like for like, but closest and van would be lighter)
public tip in my area, I have to book if I use a van and only allowed 2x a year (iirc)