oracle create tablespace without specify size question

in suse 10 , oracle 11g,

i just did a test,

create a 2G lv , raw the lv ( such as /dev/raw/raw16)

now create tablespace without specify size in this raw file,such

as

SQL> CREATE BIGFILE TABLESPACE test_big
           DATAFILE '/dev/raw/raw16' reuse;

command is ok, and select bytes from v$datafile ,check the size is 2147475456 (2147.99M），

my question is does this normal , i usually heared that thing , if datafile uesed  is lv , we must reserve 2M -10M size for LVM  management , but in our test , datafile use almost the 2G size (LV is 2G).does