Using Style Sheet for the Tab Bar
The default style sheet for the tab bar sets the minimum and maximum tab widths. The user can create a .css file and have use that as the style sheet for the tab bar. In the .css file, the widget to use is QTabBar::tab.
For more information, consider reading Style Sheets
Examples:
Change the selected tab's background to a light gray

QTabBar::tab:selected {
    background: #999999
}

Change the selected tab's text to red

QTabBar::tab:selected {
    color: red
}

All tabs will be at least 200 pixels in width

QTabBar::tab {
    min-width: 200px
}

Only the selected tab will be at least 200 pixels in width

QTabBar::tab::selected {
    min-width: 200px
}

Any of these can be combined in one file

QTabBar::tab::selected {
    background: #999999;
    color: red;
    min-width: 200px;
}
QTabBar::tab {
    min-width: 100px
}