Multicellularity and large body size of eukaryotic organisms require high metabolic rates and efficient atp production by aerobic respiration. how did bacteria change earth's atmosphere to enable aerobic respiration?

Before the origin of bacteria, it is believed that the atmosphere of earth contained negligible amounts of oxygen, and large amounts of carbon dioxide. Prokaryotes (eventually bacteria) evolved the ability to use the carbon dioxide to build cellular structures, forming oxygen in the process. Cyanobacteria are, however, photosynthesizing organisms.

so the answer to your question would be:
because oxygenic photosynthesis by cyanobacteria significantly increased the amount of oxygen in the atmosphere