You have obviously never held a Tommygun. It is lightweight, only when compared to a crew served weapon! And several of the other early SMG designs are also in the 8lb+ range.
I recently got to play with one and lightweight is not a word that would come to mind at all, nor ergonomic.
Also played with a Sten gun, and I would not use that as an advertisement for a pistol cal carbine, it was a horrible horrible weapon!