pg_attribute getting bloated in size

[Artem Tomyuk <admin@xxxxxxxxxxxxxx>]

Hi, 

i am faced with bloated in size pg_attribute table, now its 15 GB in size, from logs autovacuum is showing a lot of now removable rows:

===========>6915   2018-12-14 03:40:02 EET 00000 [7-1]LOG:  00000: automatic vacuum of table "lb_upr.pg_catalog.pg_attribute": index scans: 1

        pages: 0 removed, 1127929 remain

        tuples: 169 removed, 14656351 remain, 14501557 are dead but not yet removable

        buffer usage: 1350052 hits, 996326 misses, 20842 dirtied

        avg read rate: 19.732 MB/s, avg write rate: 0.413 MB/s

        system usage: CPU 5.61s/5.27u sec elapsed 394.47 sec

Why autovacuum cant delete those rows?

What is the main reason that pg_attribute getting bloated?

My autovacuum settings are:

log_autovacuum_min_duration = 0        
autovacuum_max_workers = 4              
autovacuum_naptime = 51min              
autovacuum_vacuum_threshold = 750      
autovacuum_analyze_threshold = 5500                                            
autovacuum_vacuum_scale_factor = 0      
autovacuum_analyze_scale_factor = 0.2  
#autovacuum_freeze_max_age = 200000000  
#autovacuum_multixact_freeze_max_age = 400000000
autovacuum_vacuum_cost_delay = 10ms    
autovacuum_vacuum_cost_limit = 1500    

Will be grateful  for any advise!