Why does backup file size become enormous when Agent schedule is changed?
You don't indicate how often your provider issues BACKUP LOG commands (which I assume are not COPY_ONLY), but here is my guess at to what is happening.

Let's assume a 'regular' transaction log backup has occurred at 8am which truncates the log.  Also, let's assume that 1gb of transaction log is being created every 15 minutes.


8:00am - Your BACKUP LOG COPY_ONLY runs and doesn't really backup
anything because the log has just been truncated by the normal log
backup.
8:15am - Your BACKUP LOG COPY_ONLY runs and backs up up everything
since 8:00am  (because that's when the log was last truncated). Let's say 1gb.
8:30am - Your BACKUP LOG COPY_ONLY  runs and backs up everything
since 8:00am (because that's when the log was last truncated) - now 2gb.
8:45am - Your BACKUP LOG COPY_ONLY  runs and backs up everything
since 8:00am (because that's when the log was last truncated) - now 3gb.
9:00am - Your BACKUP LOG COPY_ONLY  runs and backs up everything
since 8:00am (because that's when the log was last truncated) - now 4gb


You are also 'appending' this data to your existing backup because you are using NOINIT.