Because they have different resolutions. There are far more pixels on the 17" screen, so everything will appear to be smaller. Both resolution and icons sizes should be adjustable, but your native resolution is going to look much better than going back to a smaller resolution. You can however click on the desktop to move focus there, press Ctrl and scroll your mouse wheel to adjust the size of the icons if you wish.
If viewing webpages is the problem, or you have trouble with small text and if overall looks are not as important as viewability, then you can decrease the resolution. Do you know what your current resolution is? You can right click the desktop and select display settings to find out.