Hello.
Why are you obsessing about something so unimportant? You're having orgasms, and I presume you're enjoying your sexuality, so why look for something to worry about? If and when you decide to become a parent, volume isn't important either. In fact, your semen can contain the same amount of sperm now that it did when you produced more ejaculate.
I think you're worried because you've heard myths that somehow volume is associated either with pleasure or with some aspect of "manliness." Nothing could be further from the truth.
You'll find that volume ebbs and flows during your life--just as erections go up and down, especially as you age.
And if you're not getting as turned on as you used to, look at your anxiety and stress level, fatique, whether you're interested in your partner, etc. These can all contribute to a lessening of desire. And remember that desire also goes up and down, depending on other circumstances in your life. Get experimental. Find some new things that might turn you on and try them out. The main thing is to stop worrying and enjoy your life.
If you notice any drastic changes in your sexual response--such as no semen at all--then by all means, see a urologist, as this could indicate an underlying medical condition. Dr. J
The volume of ejaculate will vary according to age and your physical health,and in your case it wouldn't hurt to see a Urologist.