Winding losses in transformer
Nevertheless, it is true by experience that the primary current is
  higher when Vin is low... It implies that the volt second product is
  higher at low Vin... I have some troubles to find the physical
  relation that prove it !


Firstly, TI is talking about a regulator and secondly they are referring to (mainly) the primary current that flows due to the secondary load so, talking about volt.seconds is somewhat missing the point. The primary current is higher when \$V_{IN}\$ is lower because the "regulation controller" will increase the duty cycle to ensure the output voltage remains regulated. 

It's not 100% like in a flyback converter where increasing the duty cycle (to accommodate a greater load current or lower \$V_{IN}\$) will have a volt.second (and core saturation) implications. The article (at that point) is talking about primary referred load currents.

For a forward converter increasing the duty cycle applies more energy to the load. Yes, there will be a volt.second implication for the magnetization current but this will be much smaller than the implication for a flyback converter.

And, as the article says, winding losses will be greater at lower values of \$V_{IN}\$ - this is because the primary current is greater in order to supply the load with a regulated voltage.