Regardless of gravity, you'll have the same mass. Mass is constant, and cannot be changed by any outside factors.
There appears to still be a lot of debate within science over whether humans or climate change were primarily responsible for the extinction of many of the large mammals during this period. One hypothesis supporting the link to early human hunters is that large animals in sub-Saharan Africa co-evolved with humans as they learned how to use tools and developed ever more effective weapons. When early humans moved into Europe and Asia, taking with them their weapons and hunting skills, the populations of large animals in those areas were not accustomed to human hunting, and were therefore easy prey.
