ThomasAachen wrote:
. . .
Why are the sizes being re-calculated?
Because it may have changed since the last time it was displayed.
The size should be the same all the time, shouldn´t it?
The size of a folder isn't necessarily constant -- as you add, remove, or edit files, the total size of the contents changes.
When adds OSX them up?
Yes. That's not kept in each folder.
Happens this automatically?
No. Only when you ask for it, either by displaying a lot of things in List view, or doing Get Info.
On most folders, especially on an internal drive, it's so quick you don't notice.
But consider some of the folders, especially system folders, that contain thousands of sub-folders and tens or hundreds of thousands of files. Keeping all of them updated would take a large amount of system resources, for something you may never display.
Or folders on a network drive, where access is much slower.