Aussie here. 125's are reasonably popular for delivery riders and get amazing economy - I'm 6'2" / 235lb and managed to get a pretty easy 100mpg out of my little CBR125R I had a decade ago. It was a fun ride, didn't like going over about 65mph and topped out around 70-75mph depending on the weather, road and incline.
I've had an MC19 Honda CBR250R - yeah, the 4cyl 250 from before the 40hp japanese limit! - and it was awesome. 18,000rpm redline and got an easy 60mpg commuting on the freeway at semi-legal speeds. Really fun bike but physically very small.
The current fleet includes an XR600R (amazing bits of kit and solid 45mpg), '95 Suzuki GSXR1100 (dont ask), Royal Enfield 650 (good economy, still getting a good baseline) and then the good old MT01...
Man. Just look at it. 102 cubic inches of goodness wrapped in an R1-ish frame with an upright riding position to clear those two huge cylinders. Awful economy but Good Lord does 100hp/120ftlb feel good going down the road hahaha!!!