Well, sending more data is not a problem, and the packets are really small already. But it would also mean the XBee (this one consumes most of the power) has to wake up more often. That's basically the reason to do it that way.
A car battery... I said space is not an issue, but a car battery is pretty large though 
It's not impossible of course, but do similar devices really need that kind of batteries? You can buy plug & play temperature/humidity loggers, and they are as big as half a fist. Sure, they don't send data wireless, and their circuits are far more optimised than my hobby project. But using a car battery on the hand sounds quite drastic.