Are they purely additive, to the original monster decks? That would be a 40% increase in deck size, so it seems extreme to me, but maybe it's balanced to account for that?
(The Monster Pack rules note to "just shuffle them into the four Monster cards above the Lair card in each monster stack". And the dragon rules, which are not online, also don't mention removing anything.)
Alternatively, should some original monsters be removed, when playing with at least one of these packs?