maximizing window by double clicking its title bar (for Safari) does not fill entire screen

I am running macOS Sonoma 14.3 on a MacBook Air, and I would like for the Safari window to fill my entire screen after double clicking the windows title bar. However, when doing so, the window only expands vertically, and not fully horizontally. How can I fix this? I know for other apps, such as Discord, the window would fill the entire screen after this action. Why not Safari?