If you are considering water solubility differences between the two polymers the main thing you need to look at is how the different structures affect the overall entropy of the solution. Starch exists in general as an alpha helix while glycogen is a highly branched random structure. In the helical conformation there are less exposed -OH groups to the solution making the starch more hydrophobic than the glycogen, the end result is that the entropy of the solution is decreased because there are less free water molecules, water molecules form a shell around hydrophobic molecules most easily seen when you look at a miscle of oil floating in water. Starch is soluble in hot water because the entropy decrease is nullified by the change in enthalpy, from the change in internal energy and PV work caused by the increase in vapor pressure(although this is fairly negligable). Glycogen, having more -OH groups exposed to the solution, has more free water molecules and a higher entropy allowing it to be soluble in water a lower temperatures. Let me know if this clears it up for you. Take some time to visualize the two polymers and convince yourself of the entropy consideration. This kind of entropy consideration is one of the main contributing factors in the behavior of most biomolecules in solution.