This question already has an answer here:
I'll often highlight a bunch of files in Finder to find their collective size with Cmd+I. Sometimes the Get Info window shows the pooled size in a single window, but other times it'll open 40 separate Get Info windows. What determines when this happens?
(I've since learned that other key combinations will always show the highlighted items' collective size, but when does Cmd+I alone act one way instead of the aforementioned other?)