The Little Mac features a single beef patty, Special Sauce (also known as Mac Sauce), lettuce, American cheese, pickles, and onions on a sesame seed bun. It is basically the Big Mac minus the middle bun and a beef patty.
The Double Big Mac is the same as the Big Mac but with four patties instead of two.
McDonald's has offered a similar promotion before back in 2017 when they offered the Mac Jr. and Grand Mac for a limited time (they also brought them back in 2018). Both those burger featured bigger beef patties versus the Big Mac, however. Additionally, the Grand Mac came on a wider bun. The Little Mac and Double Big Mac, on the other hand, don't require anything that's not already in the McDonald's kitchen.
Update 3/11/2020: Prices vary but here in Los Angeles, the Little Mac is $2.59 (comparatively, a Cheeseburger is $1.49), the Double Big Mac is $5.99, and the Big Mac is $4.99.
Photo via McDonald's.