why i choose one big table for the same sets of data

– Seek time with proper index, should be exactly the same, no matter how wide”table is.
Apart from performance issues,
this will simplify design and relations with other tables (foreign keys etc).
– code little

Assuming that table B is exact copy of table A, and that all necessary indexes are created, self-join of table A should be a bit faster than join of B with A simply because data from table A and its indexes can be reused from cache in order to perform self-join (this may also implicitly give more memory for self-join, and more rows will fit into working buffers).

If table B is not the same, then it is impossible to compare.

– one more thing wordpress , drupal and joomla uses the same way