Of course size and weight matter in striking as well, but he does have a point. Just look at the champ vs. champ fights of late: McGregor vs. Alvarez, Nunes vs. Cyborg, Dillashaw vs. Cejundo, Cormier vs. Stipe – the fighter from the lighter weight class won what was largely a striking match, and I think a big part of it has to do with the heavier fighter having a hard time adjusting to the speed difference.
I'm by no means super-knowledgeable about striking, but my experience as a small non-athletic guy is that a disadvantage in reach is easier to get around than a difference in speed. Cormier does really well at HW doing just that, for example. Speed can also be dealt with, of course. The way to get around being slower is usually to make it an ugly grind, to grapple and get the clinch. But if that's not your thing, then being slower can be quite tricky. I guess one can rely on counters and timing, but that also becomes hard against someone who is faster than you. Wonderboy isn't exactly a fighter known for his dirty boxing. I can see him having a harder time with Pettis than people expect him to.