Actually yes...n thr is reason behind it...a slim phone has to pack in the same stuff in lesser space..so the battery usually employed in these cell phones are slim batteries. Now a slim battery means lesser amount of chargeable chemical (Li-on) packed into it...so tht directly relates to lesser backup..
usually if u do a quick google for high capacity mobile batteries, u'll see tht they are much more thicker and come with an extra battery cover to fit the phone specified...
So yeah, the point is, the slimmer the phone, the slimmer the battery employed and lesser the backup, more so , if the phone happens to have a large screen and extended functions..
