Proportion is about balance and how things fit together harmoniously. For example "That hound dog's tiny legs look out of proportion to that big fat body."
The prefix comes from the Latin pro meaning "on behalf of, for." If you're Proportion likes portions, and for them to be the right size. The cake's sweetness is in direct proportion to how much sugar you put in. It's a compliment to tell someone he or she has just the right proportions, but you might get slapped. It can also be a part of something, as in a significant proportion of our student body has dyed their hair purple.