I am working on a 3D mobile game for 3months and I am having a problem with my terrain making my game lag

I made a terrain for my Game and made it from 3Ds Max and is 4mb when exported as FBX file, then I export it to Unity and scale it x:80 y:80 z:80 because it is too small when I drag it to the middle, and if I build the game with it it will size up to 11mb only from 10mb without it without lag to 11mb which lags… are there anyway I could make my terrain fit with my Game? thank you!

about 9,600, is it too big for a terrain?. because i putted many models on it