Belly dumps usually add about double the regular dump box capacity.
Trailers for 35 ton trucks are usually 70 ton, 50 ton trucks run around 100 ton.
85 ton trucks have been seen with 120 to 150 ton, and 150 ton trucks get a 240 ton trailer. These are not quite double, but still a good increase, then if the conditions are right, and not much grade to overcome, they can pull doubles.
Some of the huge dirt jobs were done that way. Denver airport for one ran a fleet of 777 pulling 150 ton lead and 120 ton rear belly dumps. Same story at a large reservoir project in California. Wish I had some pics of that to share.