I've actually been through this on both sides - as a bride and as a bridesmaid. When I got married, I had found out I was pregnant very soon before the wedding and knew I wouldn't be able to buy my dress until right before. It still ended up a bit too revealing in the chest, as my breasts grew tremendously when I was pregnant.
With my brother's wedding I was losing the baby weight and I waited until the last possible second, bought a dress that was too big by the time of the wedding, going back and getting an exchange for a smaller dress. Depending on the store and if it's off the rack, you can sometimes do that. The second dress was too big by the date of the wedding, but it wasn't my wedding, so I didn't need to look perfect.
You just don't want to have a dress that is too small.