The icon used for a menu instead of a full menu is often called the ‘Hamburger’. This is used on mobile views. It can also be used if a menu is very long and busy or when a clean modern style is required.
The Divi default using the full screen view hamburger default is quite small. To increase the size I just added the following to the custom CSS area in the Theme Customiser or into the Divi Child theme CSS area.
Divi > Theme Options > General > Custom CSS