Natural breasts that are large enough will also fall to the side when lying down, but there are options to consider when implants do the same. The pectoral muscle only covers the upper portion of the implant and can actually push implants down and to the side. The skin envelope is often not strong enough to resist this (plus the weight of the implants) so the tissue thins and weakens. An internal bra using SERI Scaffold, Strattice, or GalaFLEX mesh is a very good solution for this. It appears from the photos that you may benefit from a lift as well so please keep an open mind about that option.