Presumably differentiation of labor increases the fitness of an aggregate as a whole thus implying that procaryotes are not entirely incapable of a rudimentary multicellular existence. The body also has more difficulty clearing gram -negative infections. We now know that the majority of these disease-causing microorganisms are classified as bacteria or viruses. Absolute requirements for replication are low. High surface to volume ratios.
Small spaces can support large numbers of small individuals.
The niche of small individuals is predominantly as absorbers and new niches for absorbers are found predominantly through either an increase in the spectrum of nutrient utilization or an increase in the efficiency of nutrient utilization, both consequences of increases in biochemical diversity. Short generation times result directly in rapid increases in numbers and hence large populations. Selection for short generation times provides counter pressure to selection for higher DNA polymerase fidelity, and hence for greater mutation rates. Large numbers assure relatively complete explorations of possible mutational variation by populations. Short generation times assure rapid increases in the absolute numbers of favorable mutants.