I can not figure out how to make a ProtoMech that is heavier than 9 tons. All help would be appreciated.
The short answer, at least from the books I have, is ProtoMech's cannot officially be built over 9 tons. At 10 tons they become IndustrialMechs and at 20 tons they are either IndustrialMechs or BattleMechs.
The long answer with the sources used provided below.
Checking Total Warfare (TW), TechManual (TM), Tactical Operations (TO), Strategic Operations (SO), A Time Of War, Interstellar Operations (IO) and the BattleMech Manual I found two sources that identified the weight range for ProtoMech.
1. BT TW PDF CAT 35001 2006-2013 The Topps Company, Inc. Corrected Fourth Printing. Third Printing by Catalyst Game Labs.
ProtoMech p. 187 Frenzy Attack "As noted in Physical Attacks (see p. 144), ProtoMechs cannot make such attacks. However, a ProtoMech can make a single
“frenzy” attack that combines a punch, a kick and anything else the ProtoMech can muster. The net effect of this effort is a single attack with a Damage Value of 1 for ProtoMechs that weigh two to five tons, or 2 for ProtoMechs that weigh six to nine tons."
The weight range for ProtoMechs appear to be 2 and 9 tons.
BT TM PDF CAT 35002 2007-2013 The Topps Company, Inc. Corrected Third Printing Published by Catalyst Game Lab
ProtoMechs, p. 10,—a strictly Clan invention—stand four to six meters tall and mass from two to nine tons. Larger than a battle armor suit but smaller than a BattleMech, these “mini-’Mechs” have abilities in common with both unit types.
ProtoMech Construction p. 81 Step 1: DESIGN THE CHASSIS: Choose Weight: ProtoMechs weigh between 2 and 9 tons (2,000 to 9,000 kilograms), measured in 1-ton (1,000-kilogram) increments. Within these limits, the player may choose any tonnage.
Tonnage greater than 9 and less than 20 moves into the IndustrialMech range per TM p. 66.
The maximum size of a ProtoMech appears to be 9 tons if they are being played in an official game.