Protomechs heavier then 9 tons would be ultra light mechs, since at one time mechs ran from 10 tons to 100 tons, and there are several examples from older publications like StarDate/StarDrive and Far and Away magazines that had 10 and 15 ton mechs in them. So if you want heavier Protomechs, just build a 10 ton mech or a 15 ton mech and go from there.
Darkness is a friend of mine. Sometimes I have to beat it back, or it would overwhelm me. Shirley Meier
The Wookiee, he's not wearing any pants!