Sorry for the double post, I have a new picture to upload 
By looking at my skeleton, I figure something that has to be significant :
In the animation previewer, I can look at the skeleton tab and if I select the root bone, I can see that it has a “reference Scale” set at 2.54.
It’s almost the number I found as the factor of the scaling of the pelvis bone.
I see the link between these facts
By looking at the SK_mannequin_skeleton, witch is used by the 2 SkeletalMesh, that’s what I see by selecting the root bone :
In the skeleton, the root bone has a 2.54 scale factor in each axe.
I try to set that to 1 but when I change these value, it’s a mess !
Every time I click on a box to set a value, the other value change in a very erratic way.
I finally was unable to set the scale of the root bone in the SK_mannequin_skeletal… The only way to return at the initial 2.54 value was to undo the changes I made because every time I change a value in an axe, the other axes change too into any random value.
I know that every things are linked !
The truth is certainly somewhere but I can’t see it…
Do you ?