ho my ... incredible ... the problem is my stupidity ...
The last documents i was working with has (i really don't now why ...) a resolution of 300 ppi instead of the 72 ppi i normaly use.
So there's where this problem came from ...
Sorry for the stupid question ...
Just to answer JJMAC : yes i need this resolution of 72 in my workflow because the assets i create are in 2D and in pixel perfect and not like textures apply on 3D planes for exemple.