Catastrophic Growth

Catastrophic Growth is the principle of long periods of no visible growth, followed by a brief period of intense, unpredictable growth. This is the method used by most base dragons, such as Scrappy and other draca.

Principle
"Ordinary growth" is a reverse exponential, in that it starts out being somewhat fast, and then slows down over time to a more steady expansion, dwindling to nothing. This is shown as a baby grows larger very quickly, with a few spurts here and there, but a steady addition of inches over the course of years.

On the contrary, Dragons usually go through a few stages. Over time, their cells reproduce at high rates, as in an ordinary creature, but they do not expand. Instead, they are continually compressed, unable to break through the intense strength of a dragon's scales and carapace. Once they are sufficiently compressed, a release of the dragon equivalent of growth hormone triggers the opening of their scales, which allows the cells to expand to a more "normal" size. They do this rapidly, from some "slow" dragons growing several feet per day, to others growing from 2% of their adult size to 20% in the course of hours.

For the next period after their growth, dragons are easier to kill, due to the spaces between their scales. This is why catastrophic growth happens only rarely, because otherwise it'd be easy to hunt them. They usually hide away during this period.

Implications
This means that during several scenes, Scrappy and other dragons may grow. Plot-wise, it should be connected to signficant story moments, or to add drama.

Since the scales are separated and in the process of growing to cover the extra space, dragons are easiest to kill right after a growth. Thus, there need to be injuries to any dragon who does so, or at least a hightening of risk.