KMail syncs huge amount of data - why


couple of days ago I was on a cell phone hotspot using my Laptop with KMail. Cell phone (K-9 mail) and Kmail accessing the same mail account. Monitoring the wlan0 with nethogs I could see that kmail downloads huge amounts of data for the IMAP account - some 2 GB in one afternoon, using most of my mobile data volume.
Question is why - only a few mails were added / deleted on either the cell or the Laptop.
Any chance I could Kmail tell not to sync the whole account?

Check the subscriptions for the account. F.e. enable only the Inbox en Sent items.

…which are the largest folders anyway. Thanks for the input, but I feel that will not explain or fix…