Wrap content in a vertical group with this style to get full width margins in the Inspector.
By default, content in the Inspector has a large left margin and a small right margin. For a consistent look, these default margins should be used for most GUI with regular controls.
However, some special GUI elements may benefit from occupying the full width of the Inspector, with only a small margin in both sides. To disable the default margins, override the Editor.UseDefaultMargins method in your custom Editor. Then you can wrap your GUI content inside a vertical groups to your liking. For example, you can wrap some of the GUI inside a vertical group with the EditorStyles.inspectorFullWidthMargins style and wrap other parts of the GUI inside a vertical group with the EditorStyles.inspectorDefaultMargins style.
Additional resources: EditorGUILayout.BeginVertical, EditorGUILayout.EndVertical, Editor.UseDefaultMargins, EditorStyles.inspectorDefaultMargins.