Dr. Bernstein has long argued this and documents it extensively in his book "Dr. Bernstein's Diabetes Solution"[1]. The main reason being that muscles act like natural glucose sinks that drain sugar directly out of the bloodstream, bypassing the liver, so more muscles = more glucose control.
I highly recommend the introductory chapter to "Dr. Bernstein's Diabetes Solution" by the way, even for non-diabetics. It's basically just the "Life and career" section of his wiki page, but in way more detail -- a really interesting biographical account about an industrial engineer doing diabetes self-experiments with a glucose meter he procured through his wife and going up against the medical community/orthodoxy and failing, only to finally break through when he got a medical degree late in life. I could probably upload and link to just that section if people are interested.