^ This is so not true!
In US market 1er will be offered with the same 6-cylinder(!!!) engines as 3er: 230 hp & 300hp SAE, not with some fuel-saving low-displacement I4 engines with low power output: eg. 1.2L or 1.4L engines.
The decrease in fuel consumption in the case 1er vs 3er equipped with the same engine is not that significant.
Same case in European market: best sold models are I4 diesel models (318d, 118d, 120d & 320d) - and the difference in fuel consumption between eg. 118d & 318d, and 120d, 320d & 520d is not that significant at all.
Perhaps the only effect be on psychological level: people thinking smaller cars consume much less fuel.