Memory deallocation after calling cast function

Hi

I have problem with memory deallocation. look at the following queries

1- create table test01(a) as select generate_series(1,100000000)::int8 ;

2- create table test02(a) as select generate_series(1,100000000) ;

In execution of first query, memory usage increase rapidly until the
transaction comes to end and deallocate all the memory which allocated with
palloc.
I have wondered why all the memory deallocated at the end, so the cast is
removed and query executed again. memory usage was not the same. It was
grow very slow.

I need help to find the right point to deallocate the memory,
Any idea will be appreciated.

Soroosh Sardari