Pixels - have i got this wrong?

I was doing some work testing out how different monitor resolutions play on different size monitors.

A movie that i had set the stage at 1260x862 played as you might expect on my 17” (1280 x 1024) no over lap, no problem. The same movie on my 14” monitor using the same browser settings (1280 x 1024) needed scroll bars up & side?

I thought the whole idea of pixels is that they size up or down to fit the screen size of the monitor – how come they end up scaling up so large on my 14” that they need scroll bars?