Lipids are indeed small molecular weight compounds and are present not only as such but also arranged into structures like cell membrane and other membranes. when we grind a tissue, we are disrupting the cell structure. cell membrane and membranes are broken into pieces, and form vesicles which are not soluble in water. Therefore, these membrane fragments in the form of vesicles get separated along with the acid insoluble pool and hence in the macromolecular fraction. But Lipids are not strictly macromolecules.i.e., they can also be a part of micromolecules.