Screen shrinks to the left hand side when opened

Currently trying to run on 21inch iMac. When it starts up my current screens resolution gets bigger or smaller but it zooms in anyhow. Sims 4 starts and all is okay for the loading pages until it gets to the main menu and the screen shrinks down to about a quarter of the size and moves to the bottom left. 

I'm currently running it perfectly fine on my 13inch Macbook pro w/retina but ive been struggling getting it to play on my iMac for a while now.

Please help... i recently graduated and i need something to do ☹️