Okay, lots of good thoughts here but not any that really hit on target. Yes, there is a degree of prostate involvement in ejaculate production but it basically controls when it comes out. My first guess would be that your testosterone is possibly low. Having a good doctor that not only checks your total testosterone, but your albumin, sex hormone binding globulin, and calculates your free testosterone is the key. Ejaculate is sperm and seminal fluids. Decreased volume can be a decrease in either of these contents. The average male ejaculates 1.5 to 5 milliliters (a teaspoon full) during the typical event. The bigger question is the cause for your concern. If you and your partner are having great sex and there's just less stuff coming out and no one cares, then enjoy it and go on. Typically there is not really a big deal. The frequency of ejaculation can lead to a decrease in volume for each time, but more typically is related to a lower sperm count in each episode.