This is one annoying feature: the video thumbnail is elarged on hover and it looks like this:
Situation 1: if you're not logged in
Install a cookie editor extension for your browser.
Cookie Quick Manager for Firefox
or
Cookiebro for Chrome
Use the extension to find one of the YouTube cookies named PREF. Append &f7=1 to the cookie. So volume=100 will become volume=100&f7=1. Reload YouTube.
Situation 2: if you're logged in
Go to "Settings", then "Playback and performance" and turn off "Inline playback".