#STD_FORM vs #PRIM_FORM
When a form is created, it is always based on standard form #STD_FORM (see 6.10.4 Modify Default Component Behavior). This standard form can be modified. For example the size or the visual style of the standard form can be changed, and this size and visual style become the default in all new forms.
The changes you make to #STD_FORM only control the default values for new forms. There is no inheritance. Any changes you make to #STD_FORM are not reflected in existing forms, they will only affect new forms.
Note that if you want your forms to inherit from a specific form by default, you can change the value of the Ancestor property of #STD_FORM.