I see what you are saying. But it is an illusion created by several things. 1. Most of the starfield up top is hidden, is you look around the ship in top left and compare the stars they are the same. 2. where they appear different in the top right, they are, but that is due to the nebula, and only in the nebula. It's a star nursery. 3. The stars in the middle left (above beam, below ship) aren't stars, and 4. with the beam being where it is in the photo, it does act as a divider, and really does enhance the appearance of different size stars in each side of the skybox.
But, since this IS my first skybox attempt, i'd be happy to send it to you to see what you think and get some pointers.