The physics behind the perfect size of a jet engine. Thanks to http://www.audible.com/minutephysics for supporting this video – if you try the free 30-day trial, I recommend "Ender's Game" by Orson Scott Card
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The answer to this question has everything to do with drag & kinetic energy vs momentum change (thrust) – ie, a bigger engine fan allows for a larger air mass to be accelerated a smaller amount to give the same thrust as you’d get from a smaller engine with larger acceleration. Any acceleration of air imparts energy to that air, which is in principle wasted energy, so ideally we minimize the acceleration of air while maintaining constant thrust, aka bigger engine fan. However, bigger fans have more drag, so there is an ultimate size limit.