The concept is not 100% right.

1.8 can have better FC compared to 1.6.

The math doesn't work like that.
If 1.6 is underpowered, it can consume more fuel as body weight is rather identical for both 1.6&1.8.
Unless you are comparing different car built, like 1.6 one is smaller, then different story all together.

For eg. 1.6 need 2000 rpm to achieve the same speed with 1.8 at 1800 rpm, then 1.8 may have better FC.

Also 1.8 has CVT, which have better FC than older 4 auto gear on 1.6.

1.8 vs 1.6 maintenance cost wise won't differ as they are the similar engine, engine oil need 200-500ml more?

Just CVT gear difference only.

Also do consider, 1.8 may have better second hand value compared to 1.6 due to demand wise is more concentrated on 1.8.