I second rmay's comment on scale.
Engines (perhaps even in general) do not scale down very nicely - nice meaning that you keep internal losses proportional to output (such in the case of reciprocating engines) or bypass ratios (Such as Pratt/Whitney's GTF with a bypass of 11:1 - but when scaled down, that figure dropped to 10:1 --- FTW: higher bypass means less fuel consumption).
Just remember - every 747 you see fly by is a 140KW power plant in the sky... The US commercial airline fleet has a combined output around 1 terrawatt (US power production is ~6TW)... And I'm gonna stop before I digress too much more
