Solution #1
Add this <meta> tag in the <head> of all your pages:
<!-- ... --> <head> <meta name="viewport" content="width=device-width, initial-scale=1"> <!-- ... --> </head> <!-- ... -->
Solution #2
If the first solution doesn't work, you can't fix this problem easily. The
design of your website must be updated to avoid large fixed-width elements and
to use relative CSS length units (em, rem, %, etc.)