Before this weekend, the only question about "Avengers: Age of Ultron" was how big it was going to be. Would it break the opening-weekend record set by the first "Avengers" three years ago? But now that the weekend is over, the only question about the sequel's debut is: Why wasn't it bigger?

It seems churlish to be disappointed by the second biggest three-day opening of all time, estimated at $187.7 million. But the first "Avengers" premiered with $207.4 million, and until Friday, the sequel was tracking to open even bigger, around $210 million, according to many experts and Disney's own most optimistic projections. Instead, "Ultron" opened 9.5 percent below "Avengers," whose windfall seems even more impressive in retrospect, since it occurred when ticket prices were slightly lower than they are now.

Again, if we weren't talking about sales in the $200 million range, and a movie underperformed expectations by $22 million, heads would roll.

What's behind the film's weaker-than-expected performance? So far, Disney has been blaming... Floyd Mayweather.

Yep, apparently, the boxer walloped more than Manny Pacquiao on Saturday. He also beat up on the Hulk, Iron Man, Captain America, and the rest of the Marvel heroes.

Well, not just Mayweather. Saturday was a perfect storm of high-profile sporting events. Besides the much-touted boxing match, there was the Kentucky Derby, NBA and NHL playoff games, and some big-deal baseball games, including Yankees vs. Red Sox. So that's a whole lot of eyeballs that were too busy watching sports to watch the superhero-vs.-robot competition in theaters, noted Disney Executive Vice President for Theatrical Distribution Dave Hollis.

"If not for the distractions, we would have seen a bigger number," Hollis told Moviefone. He observed that, on Saturday, the film did 31 percent of its business after 5 p.m. (at the time of peak interest in the fight and the horse race). By contrast, he said, the first "Avengers" did 37 percent of its first-Saturday business after 5 p.m., while the figure rose to 41 percent for "Iron Man 3" (now the movie with the third-biggest opening weekend ever).

Hollis said that competition from the Mayweather-Pacquiao bout came as something of a surprise, given that Disney parked "Ultron" on the first weekend of May 2015 a couple years ago, while the fight was scheduled only three or four months ago. When that happened, Hollis said, "there was an immediate recognition that this could be a distraction." And while the studio responded with more aggressive marketing, "as much as you can prepare for it, the movie being great is our best defense."

There's also some evidence that the movie skewed especially fanboy-ish. Disney polling showed that the audience was 59 percent male and 59 percent over 25. Plus, the big drop-off between Friday and Saturday (from $84.5 million to $57.2 million) suggests that the most ardent Marvel fans came out to see the film right away, and that more casual viewers didn't feel the urgency.

Hollis said, however, that the enthusiastic fan response on Friday would only help "Ultron" over the long run. The first-day filmgoers would serve as "ambassadors to spread the good word," he said. He noted that the film has received an A grade from CinemaScore, indicating very strong word-of-mouth, as well as generally positive reviews.

Indeed, the coming weeks should reveal the movie's legs. It helped that there wasn't much competition this weekend from other movies ("The Age of Adaline," "Furious 7," and "Paul Blart: Mall Cop 2" were nearly tied for second place, with only about $6 million each), and next weekend should be similarly soft, with only female buddy/action comedy "Hot Pursuit" as wide-release counterprogramming. Competition's a little stiffer the following weekend, with "Mad Max: Fury Road" and "Pitch Perfect 2" opening wide. "Ultron" won't have the easy ride that "Furious 7" did last month, but it's going to be at least two (and maybe four) weeks before it faces a serious challenge.

Plus, the "Avengers" sequel has already earned $439 million overseas, having opened elsewhere around the world on April 22. It's been No. 1 in every market so far, its worldwide total stands at an estimated $627 million, and it seems well on its way to reaching the $1.2 billion international take of "Iron Man 3," and perhaps even the $1.5 billion take of the initial "Avengers."

Still, there must be a lot of pundits with egg on their faces this weekend. Some predicted an premiere as high as $215 to $225 million for "Ultron." Hollis insisted that Disney was always projecting a debut of between $200 and $210 million. "No one was more conservative than we were," he said.

Even so, the sequel underperformed Disney's most conservative projection by $12 million. Its per-screen average, an enviable $43,886, is still $3,812 behind the opening per-screen average for "Avengers," and that movie's $47,698 average was at 2012 ticket prices. Over the theatrical life of the film, couldn't this difference accumulate into a shortfall of hundreds of millions of dollars below what Disney and Marvel hoped to gross?

"It's hard to be anything but enthusiastic and excited about where we stand," Hollis said of the "Ultron" domestic debut. "The day that $188 million is underperforming is the day to walk away from the business."