El Camino: A Breaking Bad Movie takes place immediately after the end of the Breaking Bad series finale, but in reality it was filmed almost six years later - which is why some returning characters look very different. One of the actors whose appearance changed most noticeably was Jesse's captor, Todd Alquist, played by Jesse Plemons. At the time of filming Breaking Bad's final episode, "Felina," Plemons was 24, but he was 30 by the time he filmed El Camino, and fans have commented on what they feel is a continuity error of Todd looking older and heavier in the epilogue movie.
Of course, the real explanation for Todd looking different is not only the long wait between the filming of "Felina" and El Camino, but also the fact that the Breaking Bad movie was filmed in secret over just 50 days, so there wasn't a great deal of time for Plemons to undergo any kind of radical weight loss in an effort to look like his 24 year-old self again. However, for those who need an in-universe explanation, there's actually a theory that explains not only why Todd looks older in El Camino's flashbacks, but also why Jesse Pinkman doesn't look like he's in his 20s any more (Aaron Paul was 39 at the time of filming).
Unlike Breaking Bad, which had a broader view of the storyline, El Camino is told almost entirely from Jesse's perspective. In stories like this, there's often an unreliable narrator factor, in which the protagonist's worldview and fallible memory skews what the audience sees. Although Todd was only in his early 20s in Breaking Bad, he committed some horrendous acts: murdering a little boy who accidentally witnessed the train heist; working with his Uncle Jack to keep Jesse locked up and enslaved; and, as we find out in El Camino, strangling his housekeeper to death because she came across his money stash. With all that in mind, El Camino's depiction of an older-looking Todd may be a reflection of how Jesse perceives him.
This might seem like a reach, but it's supported by the way in which Todd is introduced in El Camino (which was directed by Breaking Bad creator Vince Gilligan). In his first flashback scene, the audience sees him in from Jesse's perspective: first as a shadow over the tarpaulin that covers the cage, then through the bars of the cage, so that his face is partially obscured. The film firmly establishes that we're seeing Todd through Jesse's eyes, rather than being shown an objective view. After all, this is Jesse's memory.
From this, we can also explain why Jesse himself looks older in El Camino. A scene early on in the film, after he arrives at Skinny Pete's house but before he has shaved his face and cut his hair, shows Jesse examining himself in the mirror after taking a shower (during which he has a traumatic flashback to being hosed down during his captivity). If El Camino is being told from Jesse's perspective, then in addition to the physical trauma of being held prisoner taking a toll on his body, we are likely also seeing Jesse as he feels rather than as he is. After all the psychological damage that's been inflicted upon him, it's doubtful that Jesse still feels like a man in his 20s - and his outward appearance reflects that.
Although it could be argued that Todd's appearance in El Camino distracts from the story, it requires no more suspension of disbelief than when a character is completely recast for a sequel. Moreover, the flashbacks do add some interesting details that flesh out the character's Breaking Bad arc - for example, the fact that he kept the spider belonging to the little boy he shot as some kind of souvenir. And, of course, the manner in which he killed his housekeeper makes Todd's own death that little bit more poetic.