I don't have experience on Malaysian, but if the economy configuration downstairs is the same as Singapore Airlines, then you shouldn't assume there is any extra legroom. I was on a flight right at the back in economy last year; I had a window seat and there was absolutely no prospect of getting out of it without disturbing the people sat in the middle and aisle seats. I'm 5'3" so legroom per se tends not to be a problem for me (and wasn't) but it was far from roomy.