Strange effect… First I thought it was the box-sizing: border-box; rule that applies to about all elements in the themes bootstrap.min.css but disabling that did not change it (much).
Then I looked at the page source and spotted the issue right on the first lines:
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Move those lines to anywhere between the opening <body> and closing </body> tags. Make sure the doctype declaration (in your case <!DOCTYPE html>) is the absolute first thing any browser finds in the source code. Even a blank line before it might mess things up specially in older IE versions.
