Phytoglycogen is a highly brandied polymer of glucose that forms dendrimeric nanoparticles. This special structure leads to a strong interaction with water that produces exceptional properties such as high water retention, low viscosity, and high stability of aqueous dispersions. We have used ellipsometry at controlled relative humidity (RH) to measure the equilibrium swelling of ultrathin films of phytoglycogen, which directly probes the interstitial forces acting within the films. Comparison of the swelling behavior of films of highly branched phytoglycogen to that of other glucose-based polysaccharides shows that the chain architecture plays an important role in determining both the strong, shortrange repulsion of the chains at low RH and the repulsive hydration forces at high RH. In particular, the length scale 2 that characterizes the exponentially decaying hydration forces provides a quantitative, RH-independent measure of film swelling that differs significantly for different glucose-based polysaccharides. By combining ellipsometry with infrared spectroscopy, we have determined the relationship between water structuring and inter-chain separation in the highly branched phytoglycogen nanoparticles, with maintenance of a high degree of water structure as the film swells significantly at high RH. These insights into the structure hydration relationship for phytoglycogen are essential to the development of new products and technologies based on this sustainable nanomaterial.