These data are taken from Blattner et al. Fraser et al. A genome that size is big enough to encode enough enzymes to explain the intermediary metabolism of these organisms, and intracellular DNA concentration is certainly not a problem. Experiments done in the 1970s suggest that modern proteins might be larger than their functions absolutely require. For good measure, let us also arbitrarily dispense with 100 of the genes found in the M. genitalium genome.
Several comments are in order.
A cell that is even smaller would require even more radical departures, which I lack the imagination to envision. Second, whatever else this nanocell can do, it is not going to grow on a medium consisting of mineral salts and glucose. A biosphere consisting of organisms the size of M. genitalium or smaller looks impossible. Third, these considerations suggest that it is appropriate to scrutinize all reports of cells significantly smaller than M. genitalium with great care. Maybe cells significantly smaller than those we have been considering really do exist.