>> How was that surface constructed exactly? I see some curves in there which do not look super dense like the surface. But there seem to be some duplicates stacked up on top of each other.

The surface was constructed by lofting those blue non-dense curves. It has actually happened regularly that when lofting simple non dense curves the geometry has become dense. Must be somethign I am doing wrong while lofting etc?

Her for example I selected the 2 blue curves and just hit loft, the resulting geometry is super dense!