pg_start_backup and small max_wal_size
max_wal_size is a soft limit - under many scenarios it gets crossed. 
Specifically as for pg_start_backup it just commits and outputs the wal location - if you have archiving in place than you have nothing to worry about. 

Starting the backup will create more wal files but doesn't prevent rolling them - that's why you need the archiving as all wal files generated in the duration of the backup are needed for a successful restore. 

See: max_wal_size documentation 
Also: backup documentation