How can I keep my hair straight without it getting poofy?

I have curly hair, but when I flat iron it sometimes it gets reallllyyy poofy like i have realyyy thick hair. Is there a product or solution to this problem? I do use Frizz-ease for the frizz but its still not like smooth down it like has too much volume or something- and I just got a hair cut ( it's not that different at all than what it was, it was just a trim and it waslike this before..:) Oh and i use a CHI and I really dont wanna replace it. I also have layers but there isn't a lot of em:)