If microwave ovens are based on radio frequencies, which affects H₂O (water) at the atomic level, why does it take longer to microwave more stuff?  If I heat a mug of water to make tea, it usually takes about 70 seconds in my microwave to get it hot enough.  But if I put 2 mugs in, it takes nearly twice as long (2 minutes or more).

It’s not like the extra mug/water is absorbing the “waves” which essentially pass through them. That would be akin to saying someone would have to talk louder (or longer) in order to be heard every time another person walked into the room.