'Smaller than the SMP',
Noticeably, or you mean thickness?
Doesn't look small small
Now that I have a moment, I'll endeavor to give a better response.
Measurements aside, to me, the Sub and the SMP are not really comparable watches. Not in a quality regard, but just they are two very different creatures as far as watches go.
Granted this is 24 hours on the wrist with the sub, but my initial comparisons of the two.
My SMPs are just bigger on my wrist, the weight is more noticeable, the thickness is more noticeable, it's presence on my wrist is more noticeable. The Sub is extremely light by comparison and it is very very thin, I'm not going to say I forget it's there, because it isn't like I am constantly aware of the SMP on my wrist either. I guess I would say to me, the sub is more reserved and the seamaster pro has a more prominent quality maybe? Not sure prominent is quite the right word, but I'll go with that.
My primary point, is that opposed to my previous expectations of them being very similar almost duplicate roles in a collection, they don't. I might think the sea dweller and the SMP would be more comparable to each other as they seem very similar size wise. But then again, I didn't find the size difference between an SMP and a PO to be that noticeable, I just felt the dial of the PO was too aggressively styled for me.
Now this is coming from someone who feels a watch has to have a bezel of some kind for it to be worth wearing. The dressier and more formal omega's and Rolex's don't really appeal to me.