If you mean that it is physically too small, by just a small amount: try packing it with a piece of card at the bottom so as to push the battery up against the contacts; or get a replacement from Apple.
If you mean that it's physically too small by a large amount, something is very wrong. Ask for a replacement.
If you mean that it's too small in the sense that it doesn't hold enough charge to suit your purposes, so far as I know there's nothing you can do.
You're in the Android forums at the moment....you're welcome
, but you'll find much more help and information about iPhones here:
http://m.imore.com/
Posted via the Android Central App