Bottles now weigh less, have thinner sides, and contain less glass than they once did for a given volume. That change has been driven by manufacturing and transport economics. To
survive, the thinner bottles must be tougher than bottles produced by the old processes.
Here is my guess as to how it is done, and why bottles now bounce.
The glass used now has additives that make it better able to withstand some tension.
The glass is blown in an external mould, then at a critical time and temperature, the bottle is ejected, and the entire external surface is evenly chilled rapidly with an air blast and solidifies. The inside surface is still plastic at that time. As the inside surface solidifies, then shrinks, the inner surface is placed under tension, with the outer surface now being in compression. The glass bottle is then more difficult to break, because a crack on the outside surface cannot be easily opened by an impact, or by a surface scratch.
In effect, the bottle is now made from tempered glass, but unlike tempered window glass where both sides of the pane are under compression, the bottle has only the outer surface under compression, while the better protected inside surface is under tension.