Why the size of a table growing very fast although the records is growing normal after alter table.

Dear DBA master,

Could you please help me, I have an issue with the growing of space usage  for a table after performing reorganize the table with the following command:

alter table table_name move tablespace new_tablespace;

alter index theIndexOfThe_table_name rebuild online nologging;

My questions are:

1. Why the size of a table growing very fast although the records is growing normal after alter table.

2. How to solve this issue?

Thank you in advance.

Emal