One reason as to why there is a difference between scales is the scale seed value comes from the local value and not the relative world scale. If you create a cube primitive 12X12X12 and convert it to a poly object you loss the ability to change the local scale by changing it’s local XYZ value. If you use the scale tool what you will be changing is the scale of the object relative to world scale along with the other objects around it bt it’s local scale is still12X.
When you import into UE4 it’s the 12x value that is used and not the world scale.
To fix it before exporting hit any of the objects that you corrected for world scale, and what I do as a habit, use the xform rest that you will find in utilities and that will set the local scale to world scale