Actually radicale does not use libical, so I'm not sure to understand the connection to the libical change that you mentioned. However it is clear that the problem is linked to timezones. I cleant my calendar.ics file by removing all the custom timezones (those with "freeassociation.sourceforge.net"), and now the REPORT request transfers 1.1Mb (instead of 26Mb previously). This is still 3 times as big as the whole calendar.ics, but it is nevertheless much better. Looking at the log, it still looks that the spurious traffic comes from timezone information that is repeated for every VEVENT entry.