the hollow form has more surface area ,+ that makes it stiffer
Not in any engineering that I have seen...
In general, stiffness increases in wood as a function of the width multiplied by the square of the depth and all calculations flow from the cross sectional area of the "beam."
It is true you can lighten a structure a great deal by coring it... provided you increase the depth of the beam enough to make up the losses in cross sectional area.
I would love to see an example of a homogeneous beam less stiff then a cored beam of identical material and external dimension.
