First, you’ve selected the “Coquette” font for headings on the Appearance → Custom Design → Fonts page. That font has a lot of flourishes and is a little wider than other fonts. You also have the size on that page adjusted to +3. You can override that with the following CSS. Note that you may need to make some adjustments, this is a starting point for you:
Adjust the font-family and font-size properties as necessary.
The first block hides the search form to give the menu more space. If you decide to remove that block and bring the search form back, you should also adjust the width in the #nav-menu block accordingly.