If nothing else has changed - your diet, your exercise or activity level, stress levels, etc., this could be a hormone issue or a prostate issue.
You should see your doctor to rule out either of those - always rule out physical causes first - and if there is no physical cause, you can try some of the things listed in the article. But really, start with ruling out problems first - some can be serious, and you'll want to find out if you have those.
There is a term for this which is perceived ejaculate volume reduction which means that you are not alone. This happens to enough men that it is a medical term PEVR. Are you stressed? That's the number one reason, believe it or not. Our mind controls so much of what happens with our body. But because something like an urethral obstruction or injury could be going on, you should get checked out. 50 percent reduction is pretty huge. Do you have a regular partner?