McDonald's claims the Grand Mac features two larger beef patties totaling 1/3lb as well as a larger bun, while the Mac Jr. comes with a single 1/6lb patty and lacks the extra layer of bread in the middle. For comparison, the classic Big Mac (which remains on the menu), is made with two 1/10lb patties.
Basically, if you've ever wanted slightly more Big Mac without having to order two Big Macs, the Grand Mac appears to be your new solution. As for the Mac Jr., well, it's hard to imagine why anyone would want a smaller Big Mac, but hey, it's probably better than a plain old cheeseburger.
As for that big announcement later this month, at least there's still some hope for all-you-can-eat fries.