Downsizing Dilemmas

why can some things that work on the lab bench (fuel cells, for example) have difficulty scaling up in size?   Topic for a long post.  For fuel cells, it is about cost… piling stacks upon stacks does not allow for economies of scale.   What interests me today is downsizing things that work on big scales – what problems do we get when we try to make them small?    Case in point are gasifiers. Successfully built at grand scale (IGCC power plants), there are frightfully few examples that work economically on the small scale.  why???  I will throw out two – first, small scale systems will startup and shutdown frequently which is a problem with gasifiers that need perfect temperatures.  Second, and along the same lines, small gasifiers will behave more three dimensionally because of their limited size.   There will necessarily be horizontal gradients in temperature, for example, which can be ignored in monster sized vessels.   Others???