内存表巨占内存(288M)???

t2内存表，t1myisam(23)
insert t2 select * from t1
结果t2显示288M怎么能差这么多？？？？？？？？？mysql> desc cdb_threads;
+-----------------+-----------------------+------+-----+---------+-------
--+
| Field           | Type                  | Null | Key | Default | Extra
  |
+-----------------+-----------------------+------+-----+---------+-------
--+
| tid             | mediumint(8) unsigned | NO   | PRI | NULL    | auto_i
t |
| fid             | smallint(6) unsigned  | NO   |     | 0       |
  |
| iconid          | smallint(6) unsigned  | NO   |     | 0       |
  |
| typeid          | smallint(6) unsigned  | NO   |     | 0       |
  |
| sortid          | smallint(6) unsigned  | NO   |     | 0       |
  |
| readperm        | tinyint(3) unsigned   | NO   |     | 0       |
  |
| price           | smallint(6)           | NO   |     | 0       |
  |
| author          | char(15)              | NO   |     |         |
  |
| authorid        | mediumint(8) unsigned | NO   |     | 0       |
  |
| subject         | char(255)             | NO   |     | NULL    |
  |
| dateline        | int(10) unsigned      | NO   |     | 0       |
  |
| lastpost        | int(10) unsigned      | NO   |     | 0       |
  |
| lastposter      | char(15)              | NO   |     |         |
  |
| views           | int(10) unsigned      | NO   |     | 0       |
  |
| replies         | mediumint(8) unsigned | NO   |     | 0       |
  |
| displayorder    | tinyint(1)            | NO   |     | 0       |
  |
| highlight       | tinyint(1)            | NO   |     | 0       |
  |
| digest          | tinyint(1)            | NO   |     | 0       |
  |
| rate            | tinyint(1)            | NO   |     | 0       |
  |
| special         | tinyint(1)            | NO   |     | 0       |
  |
| attachment      | tinyint(1)            | NO   |     | 0       |
  |
| subscribed      | tinyint(1)            | NO   |     | 0       |
  |
| moderated       | tinyint(1)            | NO   |     | 0       |
  |
| closed          | mediumint(8) unsigned | NO   |     | 0       |
  |
| itemid          | mediumint(8) unsigned | NO   |     | 0       |
  |
| supe_pushstatus | tinyint(1)            | NO   |     | 0       |
  |
| cf_province     | varchar(20)           | YES  |     | NULL    |
  |
| cf_city         | varchar(20)           | YES  |     | NULL    |
  |
| cf_hangye       | tinyint(1) unsigned   | NO   |     | 0       |
  |
| uch_tid         | mediumint(8) unsigned | NO   |     | 0       |
  |
| uch_mtag        | varchar(60)           | YES  |     |         |
  |
| ismodsign       | tinyint(1) unsigned   | NO   |     | 0       |
  |
| digg_up         | mediumint(8) unsigned | NO   |     | 0       |
  |
| digg_down       | mediumint(8) unsigned | NO   |     | 0       |
  |
| uch_blogid      | mediumint(8) unsigned | NO   |     | 0       |
  |
+-----------------+-----------------------+------+-----+---------+-------
--+
35 rows in set (0.00 sec)