Just to play devil&#x27;s advocate -- density isn&#x27;t everything either.<p>Plants on Earth play an incredible optimization game.  Water is necessary not just for chemical metabolism, but because <i>throwing it away</i> is an essential part of the internal material transport systems of a plant.  Evaporation in the leaves creates a relatively lower osmotic pressure in the higher levels of the plant.  Between capillary action and the slight but insistent pull to replace evaporated water in the leaves, plants have a pumping mechanism with essentially nil energy input: minerals and nutrients gathered in the roots just ride on up with the tide.<p>Stars may be a similar story for efficient fusion.  Sure, with sufficiently advanced technology, I can make inordinately denser energy sources with less boil-off.  But do I want to?  Maybe letting the brute physics of a gravity well do the job simply has unbeatable efficiency -- just like evaporation as one end of a pump for plants on Earth is clearly no comparison to any motorized pump for volume, but for cost-effect, nothing else comes close.