e621-rescaler.user.js

// ==UserScript==
// @name         E621 ReScaler
// @namespace    bitwolfy.com
// @version      1.3
// @description  Scales up the thumbnails on e621.net
// @author       bitWolfy
// @match        https://e621.net/posts
// @match        https://e621.net/posts/
// @match        https://e621.net/posts?*
// @require      https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.1/jquery.min.js#sha256-CSXorXvZcTkaix6Yvo6HppcZGetbYMGWSFlBw8HfCJo=
// @grant        none
// ==/UserScript==

// Settings
const cropImages = true,
      thumbWidth = 280,
      thumbHeight = 320;

// Main Script
(function() {
    'use strict';

    // Reset the attached stylesheet

    $("<style>")
        .appendTo("head")
        .html(`
            div#posts-container {
                display: grid;
                grid-template-columns: repeat(auto-fill, ${thumbWidth}px);
                justify-content: space-between;
            }
            article.post-preview {
                width: ${thumbWidth}px;
                ${cropImages ? ("height: " + thumbHeight + "px") : ""};
            }
            article.post-preview img {
                max-width: 100%;
                max-height: 100%;
                width: ${thumbWidth}px;
                ${cropImages ? ("height: " + (thumbHeight - 16) + "px") : ""};
                object-fit: ${cropImages ? "cover" : "contain"};
            }
        `);

    // Rewrite the thumbnail URLs
    $("article.post-preview").each((index, entry) => {
        let $thumb = $(entry);
        $thumb.find("source").remove();

        let $img = $thumb.find("img").first();
        if($thumb.attr("data-large-url")) { $img.attr("src", $thumb.attr("data-large-url")); }
    });
})();

I did a little mod of your script...
A little less wasted space i think.
Im not coder by trade, so this probably isnt perfect, but i like it.

(function() {
'use strict';

$("div#posts-container").css({
    "display": "grid",
    "grid-template-columns": "repeat(auto-fit,minmax(300px,1fr))",
    "grid-auto-rows":"320px",
    "grid-gap":"5px",
    "justify-content": "space-between",
});

$("article.post-preview").each((index, entry) => {
    let $thumb = $(entry);
    $thumb.find("source").remove();
    $thumb.css({
        "max-width": "1fr",
        "max-height": "100%",
        "width": "100%",
        "height": "100%",
        "object-fit":"contain",
    });

    let $img = $thumb.find("img").first();
    if($thumb.attr("data-large-file-url")) { $img.attr("src", $thumb.attr("data-large-file-url")); }
    $img.css({
        "max-width": "100%",
        "max-height": "300px",
        "width": "100%",
        "height": "300px",
        "object-fit": "contain",
    });
});

})();

It's a matter of preference, I suppose.
Personally, I like having a bit of padding between thumbnails. But to each their own.

Updated my post... somehow I had an older set of code in the clipboard last time.
But thanks for looking. Just figured I'd send it in case you like it.