The more sugar, the higher the dough rose. Well, you've already read that yeast makes bread rise and become puffy instead of flat and this has something to do with yeast activity. The sugar is food for the yeast cells. The more sugar there is, the more active the yeast cells are. Yeast cells chow down on the sugar molecules, breaking them apart in a chemical reaction and turning them into simpler elements and compounds including carbon dioxide.
Carbon dioxide is a gas.
Bubbles of carbon dioxide released by the yeast get trapped in the dough as bubbles. As more and more of these bubbles build up, the dough puffs up or rises. When the dough is put in the oven and baked, the carbon dioxide vaporizes in the heat, leaving spaces where the bubbles once and giving bread its honeycomb texture.