Can someone help me understand what happened?

I have a 4-Year-Old ball python. He was a very aggressive eater when he was younger and never missed a meal. However as he grew older his appetite got smaller and the food got bigger, so I ended up feeding him 90g to 120g rats every two weeks. He did not eat every time I offered food but about once or twice a month (rarely less) he would eat.
At the start of this year he started fasting and I got so worried because he was actually losing weight. I worked on my husbandry, checked for parasites, tried different rat brands and even different water brands (was a bit desperate because I couldn't figure the cause) with absolutely no luck. I just didn't understand what happened because he had been eating these exact rats for two years. Anyway I decided to switch to smaller rats (same brand) as a last resort before consulting a vet and since I've switched he hasn't missed a meal. He eats weekly (rats weigh between 60g and 90g) and has regained all the weight he lost. I just don't understand. Why did he suddenly refuse to eat the other rats? Did I make a mistake? Has this ever happened to anyone else?
I would really like some insight if someone has some. Thanks in advance.

I think you were trying to push too much food onto him. A 4 year old male is an adult, & adult male BPs usually do best on small rats (not mediums- not what you'd expect to "fit") & not being fed too often. In the wild, they don't eat as much as they're usually fed in captivity, plus they're getting a bigger appetite in the wild than when they are laying around indoors as pets. Glad you got him figured out now.

Sounds like a typical adult male ball python. Mine get a small rat every three weeks. Bear in mind that they're not going to grow much any more and their metabolisms have slowed way down, so they don't need to eat as much or as often as when they were young.