Many environmentalists are reverse size queens -- "small is beautiful."
When Schumacher wrote the book of that title, he was responding to a real tendency to ignore diseconomies of scale -- a tendency that still exists. Up to a certain point, both organizations and physical plants produce more output for each unit of input as they grown in size. Past that point, costs of gigantism kick in, and efficiency begins to fall instead of rising.
But Schumacher assumed that this point always occurs at small or medium sizes. In fact, there are many cases in which you get economies of scale up to very large sizes indeed.
For example, computer CPUs are still made in giant factories, not neighborhood plants; your computer would cost a whole lot more if that were not the case.