I agree with the others that have said age is a major factor. I used to spurt a large quantity of cum rather forcefully and far. Now (usually, but not always), a very small amount just kind of oozes out. I first noticed a big drop in quantity after my vasectomy. I'm not sure if that actually has anything to do with it, or is just a coincidence.
However, taking the age factor out of the equation, there are still a lot of things that affect my quantity and force. All of these factors have applied my entire life, not just now at middle age. How long it has been since my last ejaculation is my biggest difference. The longer it's been since my last time, the more force and quantity. Usually anything over 2-3 days really adds to it. When I ejaculate 2-3 times in one day, is when a small amount just trickles out.
A close second factor when it comes to ejaculation quantity and force is edging. The longer I'm aroused and the sexual activity takes to reach orgasm, the stronger the ejaculation and the higher the quantity of ejaculate. This usually results in a higher euphoric orgasm as well.
Other factors that affect my ejaculation on a case by case experience: whether or not I've been drinking alcohol. My attraction to my partner, or just how turned on I am at that moment. If I'm feeling well or maybe starting to get sick. If I'm distracted or my mind starts to wander, basically, how into I am. For me personally, there are a lot of mental and physical factors (that aren't always obvious) that can affect my ejaculation.