I've been playing some GBA games on my DS and find it extremely strange that they won't use the full screen. Is there any reason why ? Any way to stretch them out perhaps ?

BTW I got my last GBA game from ebay, a rather obvious pirate version of the minish cap (It claims to a US version yet has support for French...)
Well at least the other two look legit, I'm reluctant to order any more gba games on ebay however.