Responsive Header

I wonder if someone could point me in the right direction.
A very basic question…

I have just started a new blog and am using Responsive for the first time.
The quoted dimensions for the Header image is 300 x 100 px.

Thing is when I use a header of this size, it seems very small. eg. on my laptop it is pushed over on the left hand side, with a big space occupying the right. Is this what responsive means – ie it only looks correct on a small device?