Sounds about what I'd expect from a small turbo engine - it's probably running with manifold vacuum and ~14:1 AFRs without the trailer, extra load she's probably in boost/positive manifold pressure and therefore 11:1-12:1 AFRs
We run an F250 tremor 6.7 powerstroke and a Ranger biturbo (2.0 diesel)
On the highway empty, the F-truck gets a couple of L/100 higher than the ranger which gets 10-11 L/100.
Put the same trailer behind each one, about 2.5 tonnes but 6m long, 3.2 high and like towing a parachute. F-truck gets 20 litres per 100, ranger gets 25+
Downsizing for efficiency is all well and good for something that runs lightly loaded, but work it hard and it doesn't work.