Are you coming more frequently? Volume is usually larger when there has been a longer period without ejaculation.

How is your fluid intake? Did you used to drink a lot of water, and not as much anymore, or perhaps drink less water and more soft drinks nowadays? Drinking water is very important for body hydration and naturally is used in ejaculate.

You're young yet, but diet can play a role in ejaculate quality as well. Though I wouldn't know if it would effect the volume, as opposed to just the amount of sperm.