Smalling is by far the most dominant defender we have aerially. He won 121 aerial duels last season and that was the second most in the entire league. Jones actually struggles with some aerial challenges and he likes to climb on players backs to give him spring in the jump. Rojo is slightly better in this regard but he's also been beaten easily in a few aerial duels this season or showed weakness in the challenge. They are both average in this respect.
Case in point against West Brom there were two instances where Rondon was allowed a free header with both Jones and Rojo just standing there. One of those chances had De Gea rooted and luckily it was off target. They both struggle against big and aerially strong forwards.
@Isotope and
@ZDwyr are right
Now check these stats from this season (in the league only), courtesy of Squawka comparison matrix:
Jones: 9 games played, 810 minutes, 19 aerial duels won, 67.86% aerial duels won
Rojo: 12 games played, 966 minutes, 25 aerial duels won, 56.82% aerial duels won
Bailly: 12 games played, 925 minutes, 14 aerial duels won, 60.87%
Smalling: 10 games played, 560 minutes, 19 aerial duels won,
78.08%
Smalling won the exact number of aerial duels as Jones with a much higher percentage win rate and only 6 less duels than Rojo who has played the most minutes and has the lowest percentage win rate of aerial duels. How would these numbers look if Smalling played as many minutes as the other two? Im sure one might say well Smalling played a few sub appearances at the end of games when the opposition team went route one with the crosses, but there's too small a sample of that to even make an impact. Smalling is clearly way more dominant in the air.
When I saw your response I thought you were talking about my claim about Mourinho signing mostly defenders who are 6'2 or more because that's what you had in bold letters.