How do the Dragon and Monster Packs affect Monster Deck size?
As written, they're additive.
The event pack is purely additive explicitly, lending further support for this interpretation of the monster pack(s).
Also the dragon banner is confirmed additive in this BGG thread, lending further support for the idea.
As to whether they have been balanced for that, well, that's a subjective call. In some cases, the questions of balance either don't come up or are not sufficiently weighted against other questions of marketing and so on. Whether that's the case here I cannot say.