Some organisms do all three parts and some may only do the first part, glycolysis. Explain why larger life forms do the entire process but much smaller organisms have more leeway in completing it?

This is basically because of lack of mitochondria in lower life forms.

Explanation: Glycolysis is a Anaerobic (without oxygen) process in which glucose is breakdown in smaller forms without consuming oxygen. So glycolysis is the simplest sourse of energy production and hence it is commonly seen in both lower and higher organisms. However, immediately after finishing glycolysis, the cell will continue either aerobic or anaerobic respiration, this choice depends upon the presence of oxygen and mitochondria as the further process after glycolysis i. e kreb cycle (Citric acid cycle), electron transport chain (ETC) take place in mitochondria with the presence oxygen, and lower organisms lack mitochondria hence they cannot perform process after glycolysis.

Result: Smaller organisms obtain energy only by glycolysis.