Incorrect vertices count on imported fbx skeletal meshes

Hey there

I’m facing a problem with my imported skeletal meshes.
I have a rigged asset which is 5.6k tris and 3.2k verts in Maya. When imported into Unreal 4.13.2 the vertices count shoots up to 16.8k.
5616 tris x 3 = 16848 verts.

Although, if I LOD the asset to 99.9% of it’s original polycount I get 5.6k tris and 4.3 verts (?)

I’ve played around with every single combination of export and import settings I can think of (preserve smoothing groups/import normals/triangulating the asset on fbx export etc.) yet the results are always the same.
Whether I smooth the normals or not. I’m guessing the engine is generating normals, binormals and tangents even though I ask it to import the ones attached in the file.
Is this a bug or am I overlooking something?

Currently using Maya 2017 and the latest fbx exporter.
Unreal 4.13.2