I dunno why people see it fit to make a judgement before seeing some proof. Got a lot of respect for what the NFL is but a lot of the fans just look at the numbers. All they give a shit about is how much the dude weighs and how tall he is
I know there are probably people on both sides making lame judgements about "which sport is tougher", but a balanced judgement is that both sports have an elite level that is constantly evolving to the athleticism and talent of the human beings on the field AND the requirements of the game. There isn't going to be massive difference between the two. On the rugby field, you often have to perform for 80 minutes out of 95, which means you need to last and it lends to lower bodyweights. On the NFL field, you use that unneeded endurance in explosiveness.
It's not surprising that a dude who has had a career of returning and running through lines for 80 minutes (with small halftime break) can adjust his training to be more explosive and do okay on an NFL field.
Also, not to discount his talent, but NFL seems to have really fucking low standards for kick returns. Every professional rugby league team on the planet plays a player at the back who will punish a broken defensive line on a kick