I have followed the David Attenborough series on 'First life' and heard, that an increase in the amount of oxygen in the atmosphere, as it took place just before the Cambrium, is generally favorable for life and also enables the evolution of larger organisms. Why is that so? Is it because aerobic respiration had a better conversion rate to energy?
Because it's favorable for the process of aerobic respiration which requires O2 to produce ATP, which is the little power storage unit.
With a bigger concentration of O2, it was possible to produce more ATP, should you evolve in that direction and you needed that constraint to be removed.