NUMWIDTH setting which is by default 10 and hence you can display only 10 numbers without conversion but anything beyond that becomes exponential format. SHOW NUMWIDTH will display the current numwidth in the session and SET NUMWIDTH will change this number.
SQL> show numwidth
numwidth 10
SQL> show sga
Total System Global Area 8.6973E+11 bytes
Fixed Size 37281960 bytes
Variable Size 6.3243E+11 bytes
Database Buffers 2.3676E+11 bytes
Redo Buffers 499585024 bytes
SQL> set numwidth 15
SQL> show sga
Total System Global Area 869730873512 bytes
Fixed Size 37281960 bytes
Variable Size 632433934336 bytes
Database Buffers 236760072192 bytes
Redo Buffers 499585024 bytes
SQL>