The stats show data for the mesh which includes UV data, so whether you have 1 or 2 uv sets, and also the way you lay out your UV’s can have a big impact on the size.
I guess you have the meshes as doublesided in Blender (the default). Then keep in mind that the exporter splits all the cuads in tris, and add in the UVs issues mentioned by Grant. In top of that, when you apply SDS you are actually multiplying by 2 the number of vert/faces of the mesh with each SDS level.