For me there has always been a wow factor for the big boys. A lot of people take a look at a guy like Lesnar, Overeem, or JDS and just go "wow, look at these guys, they are huge! I wouldn't want to mess with that!". It's an intimidation thing. Who would yoube more scared to be punched by, Aldo or JDS? Whether it is true or not, there is also a common conception that there are a lot more knockouts at heavier weights. Matches like Lesnar vs. Overeem sell themselves simply on the fact that you appear to have two greek gods battling. Finally, the big boys also can take the claim as being the best fighter on the planet. not pound for pound, just literally the most dangerous dude around. Lighter weight classes are great, but I think those are some of the reasons some people prefer the bigger guys.
I just wanted to comment on the whole football/soccer being entertaining for life but UFC/MMA interest waning. One thing to note is that a lot of people who watch football/soccer/hockey have a couple teams in which they are incredibly loyal too. Players on those teams will come and go over the years, but the loyalty to the team is what remains. MMA is vastly different. You have people choosing individual fighters, not an organization, as their favorites. I will never miss a canadians game, but I might miss every NY rangers game. It is possible that people are losing a bit of interest because we are kind of in a transition period where a lot of peoples favorite fighters are becoming irrelevant as the new generation of fighters steps in.
Yeah sports comparisons are bad. For football, the good teams play weekly. In UFC, good fighters fight like twice a year. People yawn when the Thursday night football game is like Jets/Jaguars, which could be like the equivalent of like an FX card. Until the UFC can throw out a big time fight on a weekly basis (aka never because thats not possible), UFC cant compete with other sports.