That is the default file size. You can increase it using the command
set db size 4096
Please note that the maximum size is 4MB. Once this limit is reached, the device will override the oldest output. The other option that you have is to put the debug to a flash drive, which will support up to 2G.
Instructions are at: https://kb.juniper.net/InfoCenter/index?page=content&id=KB12277