This is pretty much how it will work, so don’t puzzle over it.
If you want to change only one item, you can choose 2 options.
Edit the v15.master page. This is a bad idea if you have not played with the main files before. Read a lot before heading to the master page.
Edit the CSS file. This is much better, simply because rollback is easier.
Both will affect all pages that use these files.
So how do you do this?

See the class specified for the header.
class="ms-core-pageTitle"
We need to edit this in your main CSS file for your theme.

If you decide to join here, you should look for CSS that contains this class, I would choose
/_layouts/15/1033/styles/Themable/corev15.css
You can edit this using SPD or any text editor, note that the path I used is at 15 beehives on the server.
\\spserver\c $\ Program Files\Common Files\microsoft shared\ -\15\TEMPLATE\LAYOUTS\1033\STYLES\Themable\COREV15.CSS
.ms-core-pageTitle
{
font-size:2.77em;
white-space:nowrap;
color:pink;
font-size:9.77em;
}
, F5 .