The Genomes of Free-Living Spirochaetes Are Larger Than Those Living

The genomes of free-living spirochaetes are larger than those living in animal hosts. Why might this be so?

A) Free-living spirochetes will need to synthesize proteins to obtain or create their own food from the environment around them. Parasitic spirochetes can leech food particles from the animal host, and may not need to move towards those food particles.
B) The spirochetes in animal hosts are different species entirely. As different species, they would naturally have smaller genomes.
C) A smaller genome implies simplicity-the spirochetes living in animal hosts have fewer needs, so they need fewer genes.
D) It isn't so-all spirochetes would have the same size genomes, since they're all the same species of microbe.