When sugar is dissolved in water, the volume of the water increases, but not by much. Adding equal volumes of sugar and water together does not double the volume of the water, due to the properties of solutions.
The molecules in liquids have space between them. The molecules of sugar and water in solution are much more concentrated than the molecules of water alone. When sugar dissolves in water, the sugar molecules take up space between the water molecules. The mass of the total solution equals the mass of its components, so sugar water is more dense than pure water.