If one type of bacteria doubled the amount of energy it consumed, and this slightly reduced my reproductive fitness, then this type of bacteria would be better off.

It's not obvious to me that it will better off. There is a clear trade-off here, the microorganisms want to "steal" some energy from the host to live, but not too much or the host will die and so will they. I am sure evolution fine-tunes this trade-off in order to maximize survival, as usual.

The process, of course, is noisy. Bacteria mutate and occasionally develop high virulence which can kill large parts of host population (see e.g. the Black Plague). But those high-virulence strains do not survive for long, precisely because they are so "greedy".