Black smoke is unburnt fuel.
By lower bypass ratio I believe the above poster was referring to blade clearances.
Other factors such as burner section design and nozzels that spray fuel to atomize better in conjunction with higher pressure ratios burn a much larger percentage of fuel resulting in increased engine efficiency and lower emissions.
High bypass engines become more efficient and burn less fuel because a larger percentage of thrust is created by the fan. But remember this, higher tolerance engines become less tolerant, we counteract this by creating tougher materials to make these engines less susceptable to failure. Progress and failure in the desire to progress are just a factor of life.