Back up for future use. Source: https://www.reddit.com/r/youtube/comments/dt4a60/why_is_youtube_large_and_how_do_i_make_it_small/f6uutmc/

youtube_small_thumbnails.user.js

// ==UserScript==
// @name          Youtube Small Thumbnails
// @namespace     http://userstyles.org
// @description	  Youtube Small Thumbnails
// @author        one_zero
// @homepage      https://userstyles.org/styles/177035
// @include       http://youtube.com/*
// @include       https://youtube.com/*
// @include       http://*.youtube.com/*
// @include       https://*.youtube.com/*
// @run-at        document-start
// @version       0.20191108233910
// ==/UserScript==
(function() {var css = [
	"ytd-rich-item-renderer:not([is-post]) {",
	"  flex-basis: 210px !important;",
	"  margin-bottom: 24px !important;",
	"}",
	"",
	"ytd-rich-grid-video-renderer {",
	"  width: 210px;",
	"}",
	"",
	"#avatar-link {",
	"  display: none;",
	"}",
	"",
	"#meta h3 {",
	"   margin: 8px 0 8px !important;",
	"}",
	"",
	"#meta h3 a {",
	"  display: -webkit-box !important; ",
	"  max-height: 3.2rem !important; ",
	"  -webkit-box-orient: vertical !important; ",
	"  overflow: hidden !important; ",
	"  text-overflow: ellipsis !important; ",
	"  white-space: normal !important; ",
	"  -webkit-line-clamp: 2 !important; ",
	"  font-size: 1.4rem !important; ",
	"  font-weight: 500 !important; ",
	"  line-height: 1.6rem !important; ",
	"}",
	"",
	"#metadata #text.ytd-channel-name,",
	"#metadata-line.ytd-video-meta-block {",
	"  font-size: 1.3rem !important",
	"}"
].join("\n");
if (typeof GM_addStyle != "undefined") {
	GM_addStyle(css);
} else if (typeof PRO_addStyle != "undefined") {
	PRO_addStyle(css);
} else if (typeof addStyle != "undefined") {
	addStyle(css);
} else {
	var node = document.createElement("style");
	node.type = "text/css";
	node.appendChild(document.createTextNode(css));
	var heads = document.getElementsByTagName("head");
	if (heads.length > 0) {
		heads[0].appendChild(node);
	} else {
		// no head yet, stick it whereever
		document.documentElement.appendChild(node);
	}
}
})();