Just to complicate matters, it could also be true the taller gearing IS using more fuel.
When actually cruising at say 70mph 16t may well give more mph. But every time you accelerate up to a certain speed takes longer with a 16t compared to a 15/14t.Longer with the throttle open means more fuel used.( watch the live mpg on the dash for how the throttle effects mpg on a long uphill/headwind on the motorway, a 1000/1050 uses less fuel in that situation due to not having to open the throttle so much to maintain speed , probably)
Not tried 16t. But my '22 650 did 275 miles to a tank on 15t and after changing to a 14t still did 275 miles on a tank ( most I ever saw was 310 miles), but was much livelier getting up to speed. Lowering the ratio obviously helps acceleration because the revs are slightly higher when you start to accelerate (240rpm as I remember ) but because the torque curve is not a flat line it also means the motor gets to the "fun " bit past 4k,ish quicker ( edging up to peak efficiency at peak torque and less time with the throttle open again ) so as long as I keep to the same cruising speeds as before the change to 14t uses the same fuel for more performance.
I will happily caveat all that by saying I spend as little time on duals/motorways, main roads as I can feasibly get away with so others may find a different outcome