How even empty trigger increases redo generation

Very simple example:

set feed on;
-- simple table:
create table xt_curr1 as select level a,level b from dual connect by level<=1e4;
-- same table but with empty trigger:
create table xt_curr2 as select level a,level b from dual connect by level<=1e4;
create or replace trigger tr_xt_curr2 before update on xt_curr2 for each row
begin
  null;
end;
/

set autot trace stat;
update xt_curr1 set b=a;
set autot off;

set autot trace stat;
update xt_curr2 set b=a;
set autot off;
set feed off

drop table xt_curr1 purge;
drop table xt_curr2 purge;

SQL> -- simple table:
SQL> create table xt_curr1 as select level a,level b from dual connect by level<=1e4;

Table created.

SQL> -- same table but with empty trigger:
SQL> create table xt_curr2 as select level a,level b from dual connect by level<=1e4;

Table created.

SQL> create or replace trigger tr_xt_curr2 before update on xt_curr2 for each row
  2  begin
  3    null;
  4  end;
  5  /

Trigger created.

SQL> update xt_curr1 set b=a;

10000 rows updated.


Statistics
----------------------------------------------------------
         25  recursive calls
      10553  db block gets
         91  consistent gets
         18  physical reads
    3101992  redo size
        560  bytes sent via SQL*Net to client
        491  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
      10000  rows processed

SQL> update xt_curr2 set b=a;

10000 rows updated.


Statistics
----------------------------------------------------------
         11  recursive calls
      20384  db block gets
         59  consistent gets
         18  physical reads
    4411724  redo size
        560  bytes sent via SQL*Net to client
        491  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
      10000  rows processed