The original Mauler Prime designs rolling out of the factories in 3050 had less armor than a BASP.
However, by 3062 upgraded designs like the MAL-3R were featuring 16 tons of FF armor, technically MORE than the basp.
Remember that MWLL works off of 3067 tech, so most mechs are going to be 'updated' to that level in some way or other.
One of the obvious exceptions of course is the Atlas, which has been stated to be the intentional old faithful of the class, being cheaper but less world destroying as some of the other assaults.
I personally believe that the Mauler's tonnage could easily be made up for and armor added to sane levels (would anyone really take a Mauler with the armor of a Warhammer?) by remix of ballistic tonnages and the way the first time of ammo is accounted for. My guess is UAC2s aren't as stupid heavy as they are in CBT to keep them from being complete shit and useless.
I've done plenty of these calculations based off the various beta patches throughout the years and the only thing that has really gotten me to rage is the Osiris.
I could dig up the sources for where the devs said certain things that led me to these conclusions... but I'm too lazy, they're in the forums somewhere.
