There is a world somewhere between reality and fiction. Although ignored by many, it is very real and so are those living in it. This forum is about the natural world. Here, wild animals will be heard and respected. The forum offers a glimpse into an unknown world as well as a room with a view on the present and the future. Anyone able to speak on behalf of those living in the emerald forest and the deep blue sea is invited to join.
The reason why they are so much heavier is because volume increases to the cube of length, and therefore there's less of a length advantage needed than you think for a large mass estimate. Giganotosaurus could very well get up to 13.45 meters and Mapusaurus up to 13.7 meters. Based on Scott Hartman's rather underestimated Giganotosaurus (12.4 meters, 6.8 tonnes), they would be 8.68 and 9.17 tonnes respectively (already quite substantially heavier than the 7.9 tonne Sue), and with Greg Paul's corrected skeletal (12.45 meters, 8.17 tonnes), this gets higher still, at 10.29-10.87 tonnes. Now can you see how these carcharodontosaurids can be so much heavier?