[kaboom]

but if all matter that enters it is destroyed, why doesn't it stay the same size?

apologies to all for breaking from the standard humor portion of this broadcast.

[Hoplite]

I would imagine that matter is destroyed like wood is in a fire.

The wood's constituent matter still exists, just not recognizable as its original form.

The matter's mass, however, is added to that already a part of the singularity, thus making the hole bigger, if you will.

[dog breath]

It does not really destroy the matter but breaks it down and absorbs it into it's mass.