Space constraints are the energetic constraints that prevent prokaryotes from getting larger than their limit. Space constraints control the sizes of both the largest and smallest bacteria. Bacteria can't shrink more than they have already because there is not enough space left for DNA and necessary proteins. In the same way, the prokaryotes can't increase in size beyond their limit due to space constraints.
Another difference would be that DNA contains the 5 carbon sugar deoxyribose while RNA usually contains the 5 carbon sugar of ribose. One differs from the other, as ribose has an additional hydroxyl group bonded to one of the carbon atoms, where as deoxyribose does not.
