I thought it was FIXED size of 15 cards. Meaning, when you buy new cards, you replace an older one? Have I gotten this wrong?
The OL starts with a basic deck of 15 cards, which he can add to by spending XP on additional cards as the game progresses. IIRC, the preview that dealt with the OL mentioned him having the choice, at the start of each encounter, to either use his full deck as accumulated thus far or to pull out cards he doesn't want (down to a minimum of 15 cards.) So the answer is really both. You can let your deck build up so you have all sorts of options, or you can trim it down to focus on a particular strategy, and you can make this choice before each map starts.
I must confess, I'm with the original poster here. Claiming the monsters get upgraded is strictly true (from the information we have so far) - they will ALWAYS be tougher in the 2nd act, just as part of the game. This is not an 'overlord upgrade' he pays for with xp as it was in first edition with the different monster classes being upgraded.
This is true, the monster upgrade appears to be automatic (from what we know so far), but don't forget that in 2e the OL also has more freedom in terms of what kinds of monsters he uses. Sort of like the A, B, C, D choices per dungeon level in the AC game, except that he can modify his choices each time he brings in reinforcements.
There's also the question of LTs. We know they exist in 2e (and there's even three new ones to boot), but I don't think it's been made clear exactly how they will work.