It's because fastfood places need to compete on either value or quality. They can also try to do both by primarily aiming to convey quality and having a special menu or set of offerings that promise the same quality but at a better price.

Wendy's mostly brands themselves as quality focused as compared to other fast food places. So their "good deal" offering has to promise to offer the same quality at a lower price, which means smaller. So they call it big to camouflage that it's actually smaller.