Hi,
Good observations indeed 
The increase in size could be because of the change in the size of Junos image itself.
The message you are getting was added in newer versions due to some software problems reported before. If traceoptions are enable at the time of the snapshot, some members of a Virtual Chassis may go down due to possible CPU busy conditions. It is recommended to disable any type of "traceoptions" or "syslog any any" before running the snapshot to prevent any disruption.
Hope this helps.
Thanks
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