Kmail doubles hd space comparing to Thunderbird

Hello all,

Tried this question also in the dutch groups but no luck there.

Triggered by the question of somebody I also started experimenting with Kmail. I have also ignored the program for a long time. Yet I see the benefits of integration in KDE. First I want to test and put my agenda and 1 mailbox of the 5 I use in Kmail.
As I look at it now, it has benefits for me. Speed, clearer and more possibilities. But…

I was already wondering how much space Kmail would use, so before I started I checked my free space: over 18gb.
The mailbox I put in first is the largest and measures 5.8gb in Thunderbird.

After installing Kmail and the first gmail, my disk space has shrunk to 6gb …

When I go to the relevant account in Kmail, it also indicates that all subfolders contain 11.8gb !!!
That is more than double the size in Thunderbird and is only the first mailbox I use.

If this is true, Kmail is a no go for me.
Is this really the use or do I have an error setting?

Thank you for thinking along.