Brevity is the Soul of Wit: In Defense of Slim Novels

I see more agonizing over how long a novel "should be" than I feel is needed. My mystery novel (shiny new draft recently completed) clocks in at about 53k words. Short, but still above the prescribed 50k it must be to be considered a novel.
I suppose I find it frustrating when I see blog posts telling me that suspense and mystery novels should range 65k to 80k. Why? Not all agents and publishers adhere to these guidelines strictly, but it's still worrying to think that others may be discouraged by my manuscript length. Many classic mysteries like And Then There Were None, by Agatha Christie and Hound of the Baskervilles wouldn't make that 65k requirement.
I think much depends on the author's style. Do they luxuriate over scenery or stay with a starker viewpoint? I personally write dialogue heavy pieces, allowing character and plot to explain itself in interactions between the characters. I simply prefer that way of conveying information to the audience (probably leftover from all the time I spent doing theatre and working with scripts).
I think many of us can agree that quantity doesn't automatically equal quality in a first novel. In creative writing classes, exercises where you shave away all your adjectives and exposition often prove liberating. I've been in my share of creative writing workshops where a little trimming did wonders. Young writers especially have a tendency to "clear their throats" at the beginning of pieces. Their first paragraph, stanza, or chapter can sometimes be eliminated entirely. I suppose I'm trying to write as tightly as possible. I don't want to give myself room to clear my throat.
That's not to say that many breathtaking novels haven't soared over 100k words. Some stories simply call for longer books if they have complex plots spanning over long periods of time or the author has to build an entirely new world on the page (in the case of fantasy).
Through subsequent edits, I may bulk out some characters or subplots of my novel and add a few thousand words. I just don't see the point in adding bulk for the sake of it.
Never write just for the sake of meeting a word count. Unless you're just having fun with NaNoWriMo. Or meeting a requirement for one of those creative writing classes. Even then, I feel like you should be working towards something with those words.
I'm sure that as readers, we've been equally touched by a slim novel (Ahem-- The Great Gatsby) as we have by a thicker text at some point in our lives. I suppose I'm trying to convince myself that ultimately it won't matter so much. That no one will try to pigeonhole me to YA fiction if I can't break 60k. That savvy literary agents know it doesn't really matter.
It's just hard writing between the standards. I wrote a novella last winter that I'm really proud of. It was my grand experiment trying to write layers of plot and character to follow a musical pattern (with refrains, variations, and harmonies). I love that piece, but I'm not sure what to do with it.