Smallest sure, but I'd disagree that he is the weakest at this point in time, or at least when he was in full juice mode and powerlifting during one of his suspensions. Jon was a late bloomer compared to his bros. He hit his athletic stride after his high school days were past, which most certainly happens. He has the ability to pack on mass and still be functional in the fight game, it's just that MMA is such a cardio-heavy sport that that's not the best way to go about doing things.
Any way you wanna look at it, Jon succeeded in a much more demanding sport than Arthur and Chandler and it was due as much to his championship mindset and strategic brilliance as his athleticism. I have seen nothing from Arthur or Chandler to say that they could come close to their bro's accomplishments and if they were to actually fight Jonny right now they would be completely and utterly destroyed in whatever fashion Bones chose to do so. Jones is a better fighter and more complete athlete than both of his brothers in my eyes.
That's the way I see it.
In no point in time has Jon ever been as strong as either Chandler or Arthur, even when Jon was power lifting. Jon just started power lifting, his brothers have been doing it their whole lives. Squats, cleans, deadlifts, bench press, all part of weight training in Football. They've been doing it for years and both also have wrestling backgrounds, which is why they are so much bigger than him. Jon's frame is similar to his younger brother, only his younger brother is bigger in every regard (height, weight, reach, muscle, strength). & Arthur is stronger than both of them.
As far which sport is more demanding, it is harder to make it into the NFL than it is to make it in the UFC. We have plenty of UFC fighters from Football who have demonstrated that.
OSP (NFL Undrafted college linebacker)
Brock Lesnar (UFC champion/Cut from the NFL invited to NFL Europe)
Mitrione (KO'd Fedor, Undrafted NFL Special teams)
Schaub (KO'd Crocop, Undrafted cut from NFL practice squad),
Reyes (Undefeated #1 UFC LHW contender/NFL undrafted).
Now how many UFC fighters can you tell me are competing in the NFL? MMA is a sport where a football college safety like Reyes who doesn't get drafted into the NFL can train a couple of years and become the UFC #1 LHW contender and beat guys with far more years of experience than him. Now go tell a UFC fighter to start training for the NFL for a couple years, he will never even make it into the sport.
Now in terms of Jon's championship mindset and brilliance, no disagreements here. He is an incredible fighter. But in terms of their accomplishments for their individual sports, Jon is more accomplished in his sport than his brothers are at theirs. Jon is in the GOAT conversation with GSP, Fedor and Anderson. Depending on who you believe is the GOAT. Arthur and Chandler are not in the GOAT conversations for their positions although Chandler is one of the best outside Linebackers in the NFL and 2nd the NFL in sacks. But it is more difficult to become great in the NFL than it is to become great in MMA just like it is more difficult to get into the NFL than it is to get into the UFC. We already have fighters who come from football in the UFC but not the other way around.
Some context to that. The NFL was established in 1920 so the Chandler and Arthur are competing against century of athletes in their sport. MMA however was established in 1980 and UFC1 was in 1993. Jon Jones himself was born in 1987. So Jon was actually born before UFC1 and seven years after MMA was established. The sport of MMA is still in its infancy so Jon and everyone in the GOAT conversations (GSP, Fedor, Anderson) really are not competing against many fighters to begin with. The sport hasn't been around long enough and it is one of the lower paying sports. As great as Jon, Fedor, Anderson and GSP are, I doubt these fighter are the best fighters the sport will have to offer. Well that is what history in sports has shown us anyways, the greats always come later.