It's no surprise to me to see Smalling do well in Italy, as from a defensive standpoint it's a league that ranks its defenders the old fashioned way i.e. How well do they defend? I don't follow Serie A as much as I used to so maybe someone can correct me but I'd be surprised if the overall style of defending has changed in the league, the way it has in England where distribution is now such an important prerequisite for a centre back.
Smalling has always had the pace and strength to be able to deal with a variety of strikers, and at Roma he mostly plays in a back 4 where the other defenders are much more comfortable on the ball than he is, which was never the case during his time at United. This somewhat masks what I would call his main weakness, as he really was never the type of centre back you trusted to play meaningful passes. But side to side passes to either Spinnazola, Kolarov or Florenzi and letting them be more assertive with their passing is suiting him really well. Smalling is also a fair bit taller than most of the strikers he comes up against, and the Roma frontline is quite adept at pressing the opposition defence, so he is able to utilize that height advantage and win a lot of those long balls being cleared out from the back, which is obviously something a defender should be pretty good at.