The DNA is a double-stranded molecule that has a long chain of nucleotides. The RNA is a single-stranded molecule which has a shorter chain of nucleotides. DNA replicates on its own, it is self-replicating. RNA does not replicate on its own.
The most important factor that limits the size of a cell is the ability of the cell to transport materials efficiently into and out of the cell. In this case the size of a cell should have a particular form in order to export and import material.
Its an example of the predating of weaker animals.
