Hi @joe003 and welcome to RStudio Community.
In the CSS of your notebook, you set the width of the h1 title to 144%. This is the reason why it appears wider than everything else:
h1.title {
...
width: 144%
...
}
So, if you want the tabset to have the same width, you should also set it to 144% by adding this to the CSS code:
#section {
width: 144%;
}