我看到你的触发器没有问题。我测试了它，它正在工作。为了使它完整，我在表area上添加了插入触发器。

create table location(loc_id int,loc_size int);
create table area(ar_id int,ar_loc_id int,ar_size int);

delimiter //

CREATE TRIGGER after_insert_area_location
AFTER insert
   ON area FOR EACH ROW

BEGIN

UPDATE location SET loc_size = loc_size - new.ar_size WHERE loc_id=new.ar_loc_id;
-- Note: In reality, you should throw in an if statement before the UPDATE to make sure there is enough loca_size to be taken away by the ar_size.
END//

CREATE TRIGGER after_delete_area_location
AFTER DELETE
   ON area FOR EACH ROW

BEGIN

UPDATE location SET loc_size = loc_size + old.ar_size WHERE loc_id=old.ar_loc_id;

END//
delimiter ;

insert into location values(1,100),(2,75);

select * from location;
+--------+----------+
| loc_id | loc_size |
+--------+----------+
|      1 |      100 |
|      2 |       75 |
+--------+----------+

select * from area;
Empty set (0.00 sec)

-- Test the insert
insert into area values(1,2,35),(2,2,40);
select * from area;
+-------+-----------+---------+
| ar_id | ar_loc_id | ar_size |
+-------+-----------+---------+
|     1 |         2 |      35 |
|     2 |         2 |      40 |
+-------+-----------+---------+

 select * from location;
+--------+----------+
| loc_id | loc_size |
+--------+----------+
|      1 |      100 |
|      2 |        0 |
+--------+----------+

-- test the delete
delete from area where ar_id=2;

select * from area;
+-------+-----------+---------+
| ar_id | ar_loc_id | ar_size |
+-------+-----------+---------+
|     1 |         2 |      35 |
+-------+-----------+---------+

select * from location;
+--------+----------+
| loc_id | loc_size |
+--------+----------+
|      1 |      100 |
|      2 |       40 |
+--------+----------+

正如您所看到的，在插入和删除操作中，值都相应地进行了更新。