Use the adverb proportionally when you're comparing the size or amount of one thing to another. If there’s a park on every block where you live, your town has proportionally more public parks than the average town of its size.
The US Congress is proportionally representative, so states with more people get more representatives. In other words, each state gets a number of representatives that corresponds to its population. The adverb proportionally comes from its related adjective, proportional and noun proportion. Both words have a Latin root, proportionem, which means "comparative relation or analogy" and stems from the phrase pro portione, or "according to the relation of parts to each other."