Although Demons are universally dangerous, they are not EQUALLY dangerous. 

Most of the truly terrifying Demons are kept contained inside the Demonic Planes, as they do not seem to be able to come through the Fixed Portals, and so most that come through are of a "middling" danger.

However once they have made their way to the Surface they are able to grow quickly through the consumption of human flesh. 

Demons will grow physically larger if they "fill" their internal storage of energy. This can be intimidating, but actually makes them more susceptible to attacks, slower, and in some cases less intelligent. More dangerous demons will spend energy to compact their bodies and energy. The more compacted a demon has made themself, the harder they will be to harm and the more energy they will be able to access.

Even so, most demons have a "cap" on how much they can store internally. While massively beyond what any human can achieve, it is finite. 

It is said that the Demonic Masters have no such cap and have accumulated hundreds of years of energy.

Hopefully this is simply demonic propaganda, and aside from the recent incident with the Snow Queen, no Demonic Master has appeared on the surface in hundreds of years.

-Excerpt from the Slayers guide to Demonic Slaying, Revised Edition