It isn't just the rear load bay. The standard Jimny is pretty under-powered. No issue off-road but when driving on A roads at normal speeds you have to rev the engine to the point where it's not that economical for it's size. I've noticed this on little city cars. I had to hire a car to drive from Dorset to Northumberland a while back and I picked a little Toyota Aygo, because it was cheap to hire and I thought it would sip fuel. In fact, at motorway speeds I was pushing the pedal so far into the carpet that it used more fuel than my diesel van.
Add wide fat tyres to the Jimny and I'm thinking it would have better road manners and economy with a bigger and less busy diesel engine.