SQL & PL/SQL :: Redo Log Generated In Materialized View

As we know that, MV is generating more redo logs during the FAST refresh. but i need more clarifications on that.

see the below examples:

exec dbms_mview.REFRESH ( LIST => 'mv_test', method=>'F');

PL/SQL procedure successfully completed.

select a.name, b.value
from v$statname a, v$mystat b
where a.statistic# = b.statistic#
and a.name = 'redo size';

NAME VALUE
---------------------------------------------------------------- ----------
redo size 147144

see the redo size is 147144 bytes. Immediately, i refreshed in MV view. now there is no update or insert or delete stats happened in source tables. but i do see redo log generation is high for NO DATA refresh.

select a.name, b.value-147144
from v$statname a, v$mystat b
where a.statistic# = b.statistic#
and a.name = 'redo size';

NAME VALUE
---------------------------------------------------------------- ----------
redo size 42352

For no rows refresh, it takes 42352 bytes.. why oracle generated redo logs when there is no DML operations happened in source table.