Siberian tigers in the wild don't grow as much as they used to be in past (in the 1900s). Their average weight was measured 176 kg in 2005 study. But it was also said that those tigers observed in study were sick and in conflict to humans.
I think their smaller size is because of low prey density because at that time, hunting of animals that come as tigers' prey was high.
Could it also be because all the large specimens were shot?