why does an organism get larger and the sa:vol ratio get smaller?

The surface area to volume ratio is one of those inherent biological facts: as an object gets larger and does not change shape, the volume increases faster than the surface area which can cause issues when it comes to maintaining the energy required to keep the organism alive. The amount of energy needed is directly related to the organism's volume. If the volume is too large, and the sa doesn't keep up the cells will starve. The ingenious way that nature compensates for this conundrum is to change the surface area's shape (this works for organs as well). Another way it has adapated is to create whole beings comprised of separeate groups of cells that provide energy to their own specialized set of cells...which is what most animals do. Each has to manufacture its own energy and manage its own waste products; each contains a complete set of genes and manages its own genetic activity. And each uses its own surface for absorption, secretion and excretion, thus avoiding the surface area to volume problem of large objects.