I work for a company in Kansas. My annual income is 58k$ and my sign on bonus is 7000$. But I received a sign on bonus check of 4900$. The federal tax withheld was 1750$ and state tax 350$. I am on my my OPT still(F1 status). Why has so much amount been withheld on tax for students? Can I clain a portion of it back?

Also if I leave the company within a year, I have to return the joining bonus. How does this work?