No, there’s nothing magic about 40lbs. I guess if you do demonstrations in public halls it’s the sort of weight which is big enough to be impressive but small enough to carry there and back. And also small enough that you can lift it to some extent when there’s no gyro help.
At the big end they used to be used to stabilise ships. This article https://books.google.co.uk/books?id=Y-IDAAAAMBAJ&pg=PA626&dq=popular+mechanics+1932&hl=en&ei=YAAXTZCfFYfQnAeg97XJDg&sa=X&oi=book_result&ct=result&redir_esc=y#v=onepage&q&f=true describes three ship-borne gyros each of which had a 139,000lb rotor.