Why does boiling oatmeal "grow"?
Oatmeal swells when it absorbs water and is sticky enough to bind into a near-continuous mass. Water that is not actually absorbed into a grain is trapped by swollen soft, sticky goo. But it is still water, not chemically combined, and if you heat it to 100 C, it will volatilize with great expansion, stretching the goo as it does so, but the bubbles can't make it all the way out. 

In a way, it's like a slow motion popcorn, or a speeded-up rising of yeast bread. You can do something similar with a marshmallow.

I found that if you exactly repeat a recipe, the microwave timer could be precisely set to swell the oatmeal to fill the bowl, and not overflow. Then you let it cool down and do it again.