I don't know. It's sometimes easier with fingers because I don't have to worry about me coming but that is much less exciting to a girl than being filled with a dick that could explode into baby batter all over their insides at any moment.
What some girls have told me is that girth is more about them going to a fantasy place where they remember getting stuffed with dick for the first time. They do enjoy the feeling of fullness however I cannot state the relevance of girth to female orgasms.
Feeling "filled up" is another one of those triggers like the clit/g-spot/fornix. I think it is hard to make a girl cum from any one trigger alone. Your best shot is to hit as many triggers as possible in rapid succession.
It's not quite like entering a cheat code into your supermario bros remote control. It's more like just frantically mashing buttons until something happens.
Dick size does matter. Dick size is not the most important factor contributing to female orgasm. Afaik.
And slap them sometimes. They love to hate that.