What does it mean when SG says: Scrum is light weight?
I can't say if this is what the authors of the Scrum Guide meant, but I interpret it as meaning that Scrum as a framework does not describe a lot of components (i.e. roles, events, artifacts, rules). A heavyweight process framework would be one that describes many roles, events, milestones, phases, artifacts, rules, etc. Take, for example, RUP, with its 9 disciplines, 4 phases, 6 best practices, etc. If you look at the length of its documentation, it certainly looks like a heavier process framework.