"Why are large electromagnetic devices more efficient than small ones?" This was a question asked by Professor Eric Laithwaite (best known for his linear motors). To answer the question, he developed the concept of a "goodness" factor for electrical machines. His work is well worth reading.
One of his conclusions was that it was a good thing that Nikola Tesla was a confident and ambitious man. If he had decided to make his experiments with induction motors
on machines of smaller size, they would not have worked!
Laithwaite also used to refer to "paralysis by analysis" - meaning that you just had to get out into the laboratory and build it :~)