as mentioned... that's just becasue the resolution is way bigger in your photo probably. just go to image > size, and change the resolution to 72. but divergant from what's been suggested above: you dont need to res-up your smaller pic if you want to use it online. the small one you see is probably just peachy.
then double click on the meg-glass icon to fit it to 100%. double clicking the hand icon will fit it to your screen, too.
note: you can not res-up a static image with any good effect. there are programs which attempt to do so but it always sucks.
trust me: www.idreamdigital.com
also, when you cut yourself out of a photo... select yourself, then click on select > modify > feather (by 1 to 5 pixels depending on the resolution. huigher res = more pix) to avoid the hard edge. cheers.