Infobox when hovering over folder

When moving the mouse pointer over a mail folder in the folder list at 
the left hand side of Kmail, a blue infobox pops up providing some 
information about the folder in question like number of messages 
contained, number of unread messages and folder size.
That is it does so most of the time. With some folders (this does not 
seem to be related to specific folders) it tells me that it contains x 
number of messages, but the folder size is shown 0 KB, which does not 
seems correct to me. Then again other folders are supposed to have no 
messages in them, accordingly their size also nill. They definitively 
contain some messages. When I right-click on the folder icon an select 
properties I find a tab called 'properties' also, if I remember well. 
Here Kmail provides me with the size of my entire mailbox not only 
information on the mail folder I clicked on, as I would expect.
This is only a minor issues and I do not mind ignoring it. Could someone 
give me a hint whether I have some sort wrong setup or is this 
malfunctioning feature of Kmail? (I wouldn't call it a bug as it does 
not compromise the mail functionality in the slightest way)


Pascal Bernhard