[Dn_Ab]

The short answer for why dinosaurs got so large is because they could. That might seem a flippant answer but they had much less constraints than most animals left living today:

Air sacs not only allowed more efficient breathing but were also within skeletal structure, allowing for light weight strength. They also laid lots of small eggs so less reproductive investment per chick. Small heads on long necks with a sturdy body allowed long range eating without moving. With small heads and no chewing the disadvantages of long necks are well handled. For much more depth I encourage reading:

http://phenomena.nationalgeographic.com/2013/02/25/dinosaur-...

<speculative As a non-expert my own guess would be some kind of food-weight advantage feedback cycle. The larger ones were not just more protected from predation but with longer reach came wider access to edible plant material. However, the larger sauropod size, coupled with no chewing and low grade of food (and probably not stones users) required larger and larger (fermenting?) stomachs to get enough energy to support their size. (not directly related but, reasoning from whales they might have even been cancer resistant. And if you consider that their size strongly suggests a 'cold blooded' like metabolism, then given their size, sauropods could well have lived a couple hundred years) />