Making something look like it has weight is all about drawing or in this case painting its form. Not shape, but form, those are very different when it comes to drawing. You see we aren't drawing squares, circles, and triangles, we're drawing cubes, spheres, cones, and pyramids. Understanding that everything is made up of there forms and even shape is important. Shapes don't need to have depth or volume so when you treat your drawing like your drawing or painting shapes then it's easy for your drawing to look flat because it doesn't have depth or volume. However if you think of your drawing as if you're drawing forms then your image will start to look as if it has depth and volume, making it look more real. Now in order to draw these forms you do need to understand perspective because all forms exist in space. You also need to have at least a basic understanding of how lights and shadows interact with these forms in order to make your forms convincing.