Robert Pattinson in The Batman lacks the muscle that Christian Bale and Ben Affleck’s versions were known for, and the context of Matt Reeves' movie makes that the right decision. As the Dark Knight reboot's marketing campaign ramped up, it became even more clear that The Batman is a very different animal from what came before. That much had already been discussed when Robert Pattinson was cast, even despite subsequently brilliant teasers and the reveal of Pattinson's undeniably cool new Batsuit.
The Batman has faced a lot of changes throughout its development and production. Initially, Affleck was set to reprise his role from Batman v Superman: Dawn of Justice and Justice League for a solo film, but of course, the experience on the latter led to Affleck leaving the project entirely. In came Twilight star Robert Pattinson, who better-suited Matt Reeves’ darker vision, which involved reworking the film to centre on a much younger version of the character in his formative years as a hero. Naturally that meant that Pattinson's take on Batman would feel different, but there are also some distinct aesthetic changes.
It would be fair to say that there were those who doubted Pattinson as Batman from the outset. Alongside the desire from some DCEU fans to see the version Affleck wrote and a continuation of the Snyderverse Batman, there were also discussions over Pattinson’s build not fitting the image of Gotham’s vigilante. Unlike Bale and Affleck, who both noticeably bulked to play Bruce Wayne, Pattinson still appears slender, prompting claims that he is too skinny and lacks the muscle and mass to truly be the imposing figure that the Bat typically is. Given the context of this Bruce Wayne's story though, that’s not a bad thing. With Pattinson being a younger version of Bruce Wayne, his aggression and his approach to "policing" Gotham's streets will mostly be fueled by rage and his trauma. Pattinson's smaller stature makes the weight of that backstory far more impactful and removes the idea of him being a ready-made superman.
Furthermore, being muscle-bound doesn’t fit the narrative of the character’s arc in The Batman: he’s more vulnerable, but with a brutal edge to him and the contrast in his small figure and his actions is what sells his journey. Fundamentally, The Batman is also a detective film and a lot of the focus will be rooted in the mind of Bruce, rather than his physicality. His new Batsuit could balance that dynamic, with the usual bulk added by the metallic armored sections, giving the impression of a vulnerable character underneath a tough exterior. The Batman's fight sequences will also probably involve his tactical mind and technical fighting style rather than his pure brute strength, with his raw emotion also adding to his impact, as the trailers promised heavily. Pattinson’s slender frame also fits well with his Drifter persona, given how important remaining inconspicuous would be, rather than walking around Gotham like a WWE star, which Ben Affleck resembled.
Bale and Affleck’s versions of Batman are old and hardened by their time as the Caped Crusader. Affleck’s version in particular has already suffered the loss of a Robin. It makes total sense for these versions of the character to have the mass and build that they do because their intimidating figures have grown over time alongside their experiences. Pattinson’s two-year run with the cowl in The Batman fits his form just right as he is just beginning his journey. This gives him the chance to build muscle and mass over time, and grow with the Bat, both physically and in developing the character.
