This is the theoretical max. damage. It assumes the center of the explosion is exactly at the hedgehog. This can't normally be achieved because projectiles collide with hedgehogs, so in practice, it is usually lower.

But a projectile could end up inside a hedgehog with tricks like rope-knocking, or you drop your mine/grenade/whatever directly on your feet and don't run away.

You can also get higher if by random knocks you overlap a hog. Can also jump "into" a hog by jumping up if they are on a slope above you.
This is a bit harder due to an existing 0.9.25 issue on our known bugs page.

Yeah, if try harder and place mine inside the hedgehog you can get -50hp
http://hedgewars.org/node/7050
but how about other weapons like rc plane, air strike, cake? no chance to put them inside hedgehog's body

Any weapon can be placed "inside" a hog's body if the hog is rolling or outside placeable land.
For example, the wacky double height island map has an island at the top where objects are often above the placeable area. Airstriking that island will do more damage.

And rolling a hog w/ RC plane is pretty easy for extra followup bomb damage.