Totally agree with you. Mysteries should never be over 400 pages, especially not when in they are part of a series. Sequels should never be more than 100 pages longer than the first book in a series anyway, they often get too bloated with too much filler/ descriptions that take too long/ unnecessary 'side quests'.

I am more lenient when it comes to standalone books, especially Fantasy, as you know at least it's just that (unless the author comes up with a completely unneeded prequel or something -_-), but I also know I usually don't buy them especially when they are over 500 pages. Ain't nobody got time for that.