When the PSP slim was released,alot of people were regret that they bought the psp fat because from the word slim,its definitely be lighter.There are pros on cons about everything.
If you had seen the pictures above,you will roughly know what i am talking about.There is a glossy coat over the paint job, WLAN/Wi-fi switch moved to top of unit next to UMD drive. The UMD chassis has been streamlined; no awkward slide button to pop it open just a little groove you slip your thumb under and it pops out (much better). Sony has confirmed that additional RAM has been added to improve UMD load times in games. The battery is far, far lighter (and smaller). Plugging in a PSP Fat battery will give nearly double battery life according to Sony because the new PSP is far more "fuel efficient." The speakers have moved; they're close to the top of the unit, above your thumbs, and the D-pad and four action buttons have been vastly improved; they feel great, much more like PS3 controller. Includes HD video/audio out.
The psp slim is roughly 20 percent slimmer with 100 percent more roundie and thinner battery,which means lighter and about 33 percent.The psp slim's design is more streamlined but the number of smudges of fingerprints on the psp is the same.Check out the last picture,its thinner indeed.