I think it’s busier because they condense everything into shorter days and it’s harder to find times that have lulls like early morning and late night. My best Disney experiences have all been late night or early morning because of lower crowds.
I’d be interested in hearing if there is a difference in overall ride capacity now compared to 20 years ago though. Would be interesting to see just how that has changed over time. I imagine it went up despite the general forum perception.
I don't have all the numbers, but we can at least look at what the four WDW parks looked like in early 2005 vs. today. I'm just looking at the attractions that were opened/closed in that time.
For Magic Kingdom, this was pre-"New Fantasyland", so it still had 20,000 Leagues, Snow White's Scary Adventures, and Stitch's Great Escape along with a single Dumbo and Toontown Fair. It also had the "Share a Dream Come True" and SpetroMagic parades. After 2005, it gets Seven Dwarfs Mine Train, Enchanted Tales, Under the Sea, a second Dumbo, and TRON while losing the aforementioned attractions and SpectroMagic (to be replaced by Disney Starlight this summer). Taking all this into account, ride capacity probably increased at Magic Kingdom since 2005.
For EPCOT in 2005, Soarin! was about to debut, Wonders of Life was still open, and the festival circuit was a shadow of what it is today. Jumping ahead, Soarin! has a third hanger, Wonders of Life is closed, and Remy has been built. Everything else has pretty much remained the same, so EPCOT probably hasn't changed much as far as ride capacity since 2005.
At Hollywood Studios in 2005, Fantasmic! was already there, Lights, Motors, Action! was about to debut, and we had the Backlot Tour (tram only) and Who Wants to be a Millionaire. Now, Fantasmic! typically has a second performance, the other attractions are gone, and we have Toy Story Land and Galaxy's Edge instead. The overall ride capacity at Hollywood Studios is probably close to the same because the trams and theatres could hold so many people.
Finally, at Animal Kingdom in 2005, Expedition Everest was still a year away, Camp Minnie Mickey was still around, and Tarzan Rocks! was still at the Theatre in the Wild. Since, Everest has opened, Pandora was created, and Festival of the Lion King was relocated to Africa. Therefore, ride capacity definitely increased at Animal Kingdom since 2005.
Overall, that means capacity has probably increased resort-wide since 2005. What's interesting is the overall park footprints have only changed at Magic Kingdom while every other park "expansion" was a rip and replace.
While "20 years ago" is a decent chunk of time since Iger became CEO in October 2005, he can't really take credit for New Fantasyland (announced 2009), Soarin! (opened 2005), or Expedition Everest (opened 2006) since those were already well on their way in development before he became CEO, which removes a fair amount of capacity from his tenure. In all, it's probably a wash and the parks have about the same ride capacity due to Iger's leadership.