I think it would depend on everybody's body shape. I personally wore a size Small and my boobs are size D and i never had a single boob slip. Even while shooting the collection which required lots of moving around.
So i think with any item, there is no way for it to be perfect for every single person. We are all shaped so differently. You mentioned you will be buying in a bigger size, so that would mean it was already fitting slighly snug. Perhaps that could be the reason the top was pulling down. Give it a try in a bigger size and hopefully you won't experience this.