@cakewalk you have it right in your question; it is a matter of filling the pan a bit more. Nothing more, nothing less.
I failed in my last 4 attempts at pain de mie, because I hadn't saved the changes I had made, to the recipe, in my recipes book. I had used the old 500g of flour recipe, instead of the new 750g of flour recipe. I used 750g today, et voila...