Do defenders have the option to "sally forth" and engage the enemy in the field instead? This could maybe deter people from attacking with inferior numbers of higher quality troops. It could also allow defenders to engage in battle if their allies come to break the siege and counter attack the sieging army.
I like the idea of the siege minigames they added. I think if they refined the system and added different actions it could be really good. Some ideas:
1. like you mentioned, traps. Tar fields, boiling oil, spike pits, barricades, etc. With these you could funnel attackers into murder pits or outright kill them.
2. sending out raiding parties to steal supplies. This could even the odds if one side can hold out longer than the other. Its should be a gamble, so if it goes wrong, it could cost you a lot. Maybe you could even send raiding parties to open gates.
3. Tunneling under walls to crumble them.
If they invested a bit more functionality into the minigame, a siege could theoretically be won or lost before the actual battle is even fought. All in all, I agree with your assessment that sieges need to be way more expensive for attackers. It should be much harder to take a city than to defend it. If that were the philosophy behind sieging, it could potentially slow down the insane snowballing that people have been seeing.