I'm not sure to really understand you question. But I will try to give some infos
If the problem is the size of 1.2G:
You don't set minextent to 60 MB but to 60 which means when you create your table it will directly generate 60 extents. 20MB*60=1200MB is about 1.2GB.
If you want to have 60MB directly reserved for your table when you create it you should set miniextent to 3 (assuming that pctincrease=0 and next=20M).
If the problem is that it seems to generate one extent:
If you have only one datafile for the tablespace used for this table it doesn't change much if it generates 1 extent of 1.2Gb or 60 of 20Mb. But if you have several datafiles it would be surprising that Oracle creates only one extent and not one or several on each file.
The way Oracle creates the extents my also depend on the type of extent management your tablespace is using.
Hi: I create a tbs with two files both size is 40M and uniform extent is 10M, I create a table with minextents 6 on it,and the result is
SQL> select initial_extent,min_extents from user_tables where table_name='TEST1234';
so the extent is cross two files!