You see what Ryder did? Kessler clear decision. Smith is massively overrated and only has size as a true advantage. For world level, his skills are basic, and Kessler is one of the best you can get at 168 lbs.
I was going to suggest it due to the class difference, but I think Smith's sheer size helps him in that department.
Figured that was where you were coming from, and I don't categorically disagree, but then I think of those moments late where Ryder's punches seemed to bother him some and I can't help imagining what the viking's punch selection, accuracy and way above average pop would do to him.
I agree with the stoppage for my boy Mikkel. Smith would get outboxed and tagged along the way, even worse than he was against Ryder. Late on Smith will get frustrated and tired, like he did against Ryder and start swinging wide and lunging in. This is where Kessler will finish him off.