Basically, the important point is that the surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume. What if a cell lost its cytoplasm. Without the cytoplasm all the organelles would move around and damage the cell. Without the cytoplasm giving the cell shape and support the cells would be weak. Surface area in a cell is important because the surface of the cell is where the gets all nutrients it can't produce itself, and releases all waste products.
