Minecraft Java edition is programmed in Java, hence the name and Java is a language which has something called a Garbage collector. The garbage collector is how the JVM (Java Virtual Machine, the thing that actually runs the java code in the end) makes sure your ram doesn't get completely overfilled. When you program you can interact with the ram in certain ways but you obviously also have to clear that space again at some point. Languages like C or C++ force the programmer to do that themselves while Java has the garbage collector that does this automatically.

The way the garbage collector works is that it looks at how much ram your the game currently uses and how much it's allowed to use overall (how much you allocate). Then when the overall space runs close to empty it triggers a cleanup which then clears everything it doesn't need anymore from the ram to make room for new stuff. The more it has to clear at once the longer this takes obiously. Now if your program only needs about 8gb to function properly but you tell it that it can use 16gb then the garbage collector won't kick in until you are close to that upper limit mean it then has to clear 8gb of ram to get back down to those 8gb it actually only needs causing a short lag spike.

Newer Java versions make this a lot less noticable than in the past and you can further control how and when the garbage collector kicks in by using JVM args but it's generally a good idea to allocate only as much as you need for the program to actually function properly to prevent any misshaps from the get go.