I’ve seen vertex count increase in various game engines upon import, but not exactly triple the amount.
Normally, you see verts increase where you have UV seams/islands, normals that are not shared, smoothing groups, etc, there’s also export options that can impact vert count in 3dsmax (ie split per vertex normals).
You might want to investigate further and play with export(3dmax) and import(UE4) options. From the pictures provided, my guess would be due to normals being imported or exported on the dome, in 3dmax it looks smooth where in UE4 you can see individual faces, might be that split vertex normal thing?