This is one of the most important functional problems of life. You'll investigate this in one of your lab exercises. The problem is that surface area is a function of the square of the length of an object, while volume is a function of the cube of the length. That means that, as an object gets larger without changing its shape, its volume increases at a faster rate than its surface area. If you consider one simple task of survival, it becomes obvious why this is a problem.
Cells get nutrition by absorbing it directly through their surfaces.
Thus, the speed at which a cell can feed itself is directly related to how much surface area it has. But the amount of cell that must be fed is determined by its volume. If the volume gets too big, and the surface area doesn't keep up, the cell will starve because it won't be able to get enough nutrition to feed all of itself. There are essentially three ways around this difficulty. One is to change shape by flattening the object.