Slipping in a buttplug or fingers will put pressure on the prostate and urinary tract. A similar thing happens when you mash your thumb over the opening of a freely flowing water hose. It makes the water stream shoot farther and harder. Now for quantity. This depends on the individual, his age, and how long it's been since he's last ejaculated.
Some guys come a little, some come a lot, and age has a lot to do with the amount of ejaculate.
Instead of saving up semen in order to spurt a bigger load, I'd suggest coming on a regular basis. I'd pay more attention to the quality of your orgasms and the pleasure you get in sex with yourself and a your partners.