JDS had a decent reach advantage on Nelson. He could use his mobility and angles to keep Roy off him and maintain a distance where he could hit Roy but Roys arms wouldnt be long enough to hit him.
Carwin is the opposite. He has reach on JDS. JDS will need to come forward to get inside Carwins range. JDS may not have the distance he enjoyed against smaller fighters like Roy that he can utilize to make himself look pretty. JDS needing to get inside sets him up nicely for a shot even if Carwin enjoys plodding moreso than exploding forwards.
Carwin also seems to have excellent accuracy and timing with his punches even if his mobility has never really been displayed I think Carwin can move much faster when he needs to.
Thats how I see it, anyway.
I think the big difference maker there isn't the reach IMO. Look at how the two fared against their common opponent in Gonzaga. Carwin was almost KO'd by Gonzaga (a man with less reach than Junior) while JDS just tooled him all over the place. Carwin tends to eat a lot of punches so he can get in and chuck bombs, which JDS won't let him do. JDS will use his footwork, move around Carwin instead of standing right in front of him and punching like Gonzaga, and tag him on counter shots like he's done to all of his opponents. JDS does NOT get hit very much because of his head movement and footwork.
Also, the reach advantage you're talking about is only 1 1/2 inch. Remember, they measure knuckle to knuckle so Carwin's true reach advantage is only half of the difference between the two. Add to that that the reach advantage he DOES have is all coming from his shoulder width, not his arms, and it's a far less pronounced advantage. Carwin will have to stand perfectly perpendicular to JDS to have that reach advantage, meaning he'll really only get it on his jab. Carwin's KO power comes from his right hand, not his left. I also don't think he has the striking technique to really utilize such a small reach advantage to begin with. Factor all of that in and that reach advantage becomes basically nil.
If this fight stays standing, JDS wins handily outside of getting clipped by a big power shot from Carwin which could very, very well happen. There's no way he'll get out struck by Carwin.
Now, if this fight goes to the ground then that's a wholely different story and the fight becomes much more competitive. We've only heard of how good JDS is off his back, we haven't yet seen it. We have seen, however, Carwin's takedown power and it's going to be very hard for Junior to stop it. That's what this fight comes down to IMO. Can Junior maneuver his way around Carwin and keep from getting taken down? If so can he have an effective offense while doing so? If he stuffs Carwin can he get out fast enough to avoid those huge bombs Carwin throws inside? Those are the three keys to this fight. I believe JDS can avoid him long enough for Carwin to make a mistake so JDS can put him out with something or at least rock him and get him to gas himself.
Either way, I still put this fight in my mind at like 60/40 JDS.