We have two goals here: Make the girls look good, and support 'em. So. You want to find a maxi dress that will allow you to wear a nice supportive bra underneath, AND one with a deep v-neck. A V-neck cut is more slimming for those "extra lucky" in the boob department. The deep V-neck will also show enough skin to keep the whole look slimming and sexy, and not like an aged Grandmother celebrating her 50th wedding anniversary in a Hawaiian themed muumuu (Grandma got away with it because she was 76. When I'm 76 I will do the same. But not until age 76).
Below left is the Sweet Pea by Stacy Frati Empire Dress in purple floral, $128 at Nordstrom.com. I love the color, I love the neckline, and how the cut just skims over the model's curves. She can also wear a bra underneath. It's perfect. This is one of those dresses that could easily be worn out and about with the kiddo, or to a summer wedding.
Below right is the Jersey Maxi Dress by Boden, $88. Again, the neckline is nice and deep, but still supportive (and bra friendly), and the skirt skims nicely over curves and poochy stomachs. I don't love the length, but I suspect that the model is very tall (and in heels), so it would probably be fine for most of us.
When talking about boobs, there are different definitions of "big". If you have big boobs, but are still comfortable in a strapless bra, then the below look from Victoria's Secret is for you. The Maxi Dress, on sale for $99, will show off a tiny waist, skim over hips, and still shows enough skin to be sexy.