Whether elastic has set with default Xms and Xmx or should I always remember to set it it in environment per node?
I'm asking because I'm observing some wire increase heap on nodes, and I think this graph should look more like saw teeth. Why this memory is not being dumped?
In the past it was best practice to set your xms and xmx, but hasn't been for awhile (looks like it might have changed with 7.11), so the answer to your first question is not anymore. See heap size settings for more info.
While digging through the documentation I see that in version 7.11 the garbage collection settings will change behind the scenes when using heaps <8gb. 7.11 changes. This is a total guess on my part but maybe could explain the behavior you are seeing.