Curious size variations

[JEAN LUSETTI]

Hi,

I am facing curious storage differences in size for 2 similar tables:
CREATE TABLE TSENSOR (
DateTime DATETIME NOT NULL,
SensorID TINYINT UNSIGNED,
Value FLOAT,
PRIMARY KEY(DateTime,SensorID));

CREATE TABLE SENSORS (
DateTime DATETIME NOT NULL,
SensorID SMALLINT UNSIGNED,
Value FLOAT,
PRIMARY KEY(DateTime,SensorID));

Only difference is SensorID as TINYINT in TSENSOR and as SMALLINT in SENSORS, should be only 1 byte more per record for SENSORS.
Indexes have been created identically:
CREATE INDEX SensorID ON TSENSOR(SensorID);
CREATE INDEX DateTime ON TSENSOR(DateTime);

CREATE INDEX SensorID ON SENSORS(SensorID);
CREATE INDEX DateTime ON SENSORS(DateTime);

However, for similar number of elements, table size is very different (more than 4x):
SELECT COUNT(*) FROM TSENSOR;
+----------+
| COUNT(*) |
+----------+
| 3308886 |
+----------+

SELECT COUNT(*) FROM SENSORS;
+----------+
| COUNT(*) |
+----------+
| 3581182 |
+----------+


source /home/pi/mysql/dbsize.sql;
+---------+-----------------+----------------+-----------------+
| Table | Total Size (MB) | Data Size (MB) | Index Size (MB) |
+---------+-----------------+----------------+-----------------+
| SENSORS | 511 | 390 | 121 |
| TSENSOR | 111 | 61 | 50 |

dbsize.sql is as follows:
SELECT
TABLE_NAME AS `Table`,
ROUND((DATA_LENGTH + INDEX_LENGTH) / 1024 / 1024) AS `Total Size (MB)`,
ROUND((DATA_LENGTH) / 1024 / 1024) AS `Data Size (MB)`,
ROUND((INDEX_LENGTH) / 1024 / 1024) AS `Index Size (MB)`
FROM
information_schema.TABLES
WHERE
TABLE_SCHEMA = "t_monitoring"
ORDER BY
(DATA_LENGTH + INDEX_LENGTH)
DESC;

So basically, each record (based on Total Size) in TSENSOR is about 34 bytes whereas it does about 154 bytes in SENSORS.

I populated data from TSENSOR to SENSORS (I don't think it really matters here but to make sure) using:
INSERT INTO SENSORS (DateTime,SensorID,Value)
SELECT DateTime,66*256+SensorID,Value
FROM BSENSOR;

INSERT INTO SENSORS (DateTime,SensorID,Value)
SELECT DateTime,84*256+SensorID,Value
FROM TSENSOR;

Does someone have an explanation?
Thanks,
Jean