  Experiments show that macromolecular crowding modestly reduces the size of
intrinsically disordered proteins (IDPs) even at volume fraction ($\phi$)
similar to that in the cytosol whereas DNA undergoes a coil-to-globule
transition at very small $\phi$. We show using a combination of scaling
arguments and simulations that the polymer size $\overline{R}_g(\phi)$ depends
on $x = \overline{R}_g(0)/D$ where $D$ is the $\phi$-dependent distance between
the crowders. If $x\lesssim \mathcal{O}(1)$, there is only a small decrease in
$\overline{R}_g(\phi)$ as $\phi$ increases. When $x\gg \mathcal{O}(1)$, a
cooperative coil-to-globule transition is induced. Our theory quantitatively
explains a number of experiments.
