Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
create table demo pctfree 0 as with q as (select 42 x from dual union all select 42 from dual) select 42 x from q a cross join q b cross join q c cross join q d cross join q e cross join q f cross join q g cross join q h cross join q i cross join q j cross join q k cross join q l cross join q m cross join q n cross join q o  cross join q p  cross join q r cross join q s cross join q t  cross join q u
1048576 rows affected

select dbms_xplan.format_size(bytes) "size" from user_segments where segment_name='DEMO'

size

12M

-- adding a column does not add space :)
alter table demo add b numeric ;
select dbms_xplan.format_size(bytes) "size" from user_segments where segment_name='DEMO'

size

12M

-- even with a default value in it :))
alter table demo add c numeric default 42 not null;
select dbms_xplan.format_size(bytes) "size" from user_segments where segment_name='DEMO'

size

12M

-- proof that table would grow if it updates the rows
update demo set b=42;
1048576 rows affected

select dbms_xplan.format_size(bytes) "size" from user_segments where segment_name='DEMO'

size

12M

-- proof that table would grow if it updates the rows
update demo set b=43/3;
1048576 rows affected

select dbms_xplan.format_size(bytes) "size" from user_segments where segment_name='DEMO'

size

12M