I'm a concave (saggy) 36 C/D and it fits with no spillage.  The strap connectors do fall out from the insert place in the cups but I reinforced them.
I thought it would be too big when I ordered bases on reviews but it's not.
 I did buy an extra halter strap https://www.amazon.com/gp/product/B01EUDOZTO/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1on Amazon which really makes a difference ( I got a medium, could have used a small).  I haven't gotten it wet yet so I can't speak to the heaviness issue.  Tying the main band it tight helps a lot.