Unity increase the triangle count of imported mesh models. why?

Hello Freaks, whenever I try to import a 3d model in unity. Its takes triples the times of triangle count. I need to know. how to Optimize this for better performance and rendering.

Different applications store vertex attributes in different ways.

Unity needs separate verts for different attributes. Which can be colours, normal information for smoothness/hard edges, UVs, and so on.
Other applications may store this information as a singular merged vertex, and so your geometry details may be off between applications.

Unity also requires geometry be triangulated, creating more vertices in Unity than may be displayed in your modelling application.