The size of a node on your Map indicates the size of the content of the file/notebook/tag. For example, in the image below, the video files on the left are much larger in file size compared to the documents on the right:
Here's another example - in the image below, there are many more notes assigned to the 'lunch' tag than the 'breakfast' tag. Therefore, the 'lunch' tag appears much larger in size:
Note: the sizes of folders and notes are not always consistent with their actual size. This is because for folders, we do not know how many files it contains until it is opened; and for notes, we have access to the amount of text a note contains, but the amount of a text a note has is not always representative of other content in the note (eg. images).