It sounds like the divisions emerged organically and intrinsically from the story – that's how it should be. Don't worry that some are long and some are short. That's not a flaw.
Forcing the story to fit a rigid, arbitrary amount of pages – like a screenplay that must introduce pre-requisite conflicts at "percentages" of running time to fit cinema turnover schedules or TV commercial breaks – is much likelier to lead to narrative flaws, even if clever writers are good at constructing stories that make such contrived "conflict beats" seem natural.
Structure should fit the story, not the other way around
Book chapters are not TV episodes. Novels are not Hollywood screenplays (hopefully). Most readers will not conform their reading sessions to these structures, the way it would work with timed-media where viewers are passive to external commercial forces like distribution and opening weekend box office returns. Sure, novels can be structured this way – like a movie or TV episode – but that only makes sense if the goal is to "dumb it down" for readers programmed by mass-media entertainment, or if the author hopes to sell the novel as a treatment for Hollywood. Novels are more often about characters and ideas, movies are about action and conflict. In timed-media, character development and ideas are secondary to pacing. That's not right or wrong, it's just a different narrative medium.
Chapters denote character turns and story progression
I would call the shorter divisions "chapters" – although there is no obligation to number them or label them.
Since they emerge from the story, rather than the other way around, it's likely the author is using the divisions to punctuate character "turns" and story development, not contrived conflicts aimed at the easily bored, impatient attention spans of passive viewers. Conflict-narratives are made more complicated, and resolved, within a set conflict-arc. Character narratives are not so neat, and in most cases rich personalities do not "solve" their flaws with a pat psychological self-realization at the midway point, or a simple conflict resolution at the top of Act 3.
A chapter might be short because that's where the emphasis of change needs to be. Character turns are often subtle and incomplete, so a non-diegetic chapter break can can be a narrative tool to cue the reader that something significant has occurred, like a pregnant pause or a fade to black. They have a moment to stop and consider what has just happened.
Sections are bottlenecks
I would call the longer divisions "sections" but again there's no obligation to label them at all.
Since they (roughly?) reflect locations, and the story is about world-hopping, it makes sense to name these longer sections after their locations. It gives a feeling of importance to the structure, like a point of no return. Characters cannot go back and "fix" an earlier conflict on another planet, that opportunity has passed. The story moves on and leaves certain elements and story threads behind. A tonal shift, or re-focusing of the story (characters and goals) seems likely at these bottlenecks. For the reader it's a signal that everything has changed as a new phase begins.
2001: A Space Odyssey is a very clear example. It breaks its story into "chapters" that progress the character and conflict, and "parts" that are clearly labeled and are profoundly different in tone, cast of characters, and story goals.
Call the shorter divisions "chapters" and allow the book to present those emphasis markers where necessary.
Name the longer sections after the planets – or a similar neutral/factual term that signals the last "world" has been left behind and a new one begins.