thats probably because mpg is proportional to power produced
not completly, efficiency is important.
you can get higher powered engines with better mpg than other equivelents.
306 d turbo - 1.9td ,90bhp, 45mpg
megane dti - 1.9td, 100bhp, 55bhp
200 sdi - 2.0td, 105bhp, 50mpg