Each manga volume adheres to a page limit set by the publisher, and it will never go above or below that. That's why some manga will have paddings in shape of character illustrations, glossary, or a simple title drop... while others will be crunched so hard that there's only 6 extra pages added to manga volume compared to the magazine raws.
Note that this is just my personal observation, so it might not be what's actually happening in the field.