[dangrossman]

You're not committing to a usage level, you're committing to a pricing level. If you reserve a small instance, you get a small instance, no matter what utilization level you choose. It's the same exact resources no matter what you pick.

The utilization levels are pricing tiers:

Light utilization = lowest upfront cost, highest hourly rate.

Medium utilization = medium upfront cost, medium hourly rate.

Heavy utilization = highest upfront cost, lowest hourly rate.

The names are meant to signify the trade-off you're making. If you run your instance only an hour a day, you will pay the least by choosing "light utilization": the hourly cost is high but you're only going to multiply that by a small number, so the savings in the up-front cost will dominate the total cost. If you run your instance 24 hours a day, then the hourly rate will dominate your total costs, so you'll save money by choosing "heavy utilization" with a higher up-front cost but lower hourly cost.

Segmenting the costs makes the pricing table more difficult to read, but it optimizes for everything else: you pay the lowest possible price for guaranteed resources, and Amazon has better knowledge of how much spare capacity it actually needs to handle the reservations.