That's because All of those players are over 6'0 and have long limbs, for e.g there is only about a 5-7kg weight difference between Niu and Turbo (6'4) but Niu looks more jacked due to being 5'10 but you'd definitely say Tom is more powerful yet looks like a praying mantis because of his body shape. (At 6'4 and 102kg you won't look like a body builder). In terms of agility and fitness I'd have to disagree with you, Niu is probably the best on his feet at the club his ability to make people miss is quite incredible, he bulked up to play centre this year and I think he'll only lose a couple of kgs if any at all because he seems to get through his work pretty well atm. He has the right balance between size, speed, footwork and he doesn't get swallowed in his kick returns (like last year) due to that balance. He needs to improve his defensive positioning and work on his catching ability more then anything.
I think Tesi is very close to having all the attributes we'd want from a fullback in attack, but I'd hope to see a bit more strike from his running game.
You can see that there is some ball playing there with some of the cut out balls he's thrown, but he does choose to run more often than not... so executing those 3v2's still needs some work, but it's not really the hardest thing to teach and when his reading of the defence gets better I think he has the skill to execute cut out balls, probably not as good as Ponga, Walsh, etc. though
He also puts in nice grubbers off the back of sweeps... if he could improve his tackle busting and strength in his running game he'd be right up there.
He is only 19/20 so hopefully he fills out a little as he's getting older and gets that bit more power to his running, because then he brings all the threats you want from a fullback sweeping around the back and also supporting through the middle (inside ball off the ruck would be back on).
He does need to work a lot on his defence though, and really needs to work on his urgency to get across on the goal line defence.
We have Darius there who is regarded as a great defensive fullback, but I dont really know if his style of defence is inline with what a fullback is required to do nowadays.
When Darius was fullback we quite often got caught out from kicks in behind because he was defending up in the line and not dropping out to float in behind once it got away from the ruck.... but that may be more of a fitness thing and potentially Darius was becoming too "broken down" at that point and just not able to float in behind the line from side to side like a fullback is required to do now.
It would be interesting to go back and watch some games of his before he did his hammy at the end of 2017 (I think??)... that seemed to be the turning point in his form. Prior to that he may have been doing the covering from side to side and if that's the case then he should be doing 1v1 on our fullbacks for the entire off season
