SELECT meta_key,
COUNT(*) AS `Rows`,
ROUND(
SUM(
LENGTH(
CONCAT(meta_id, post_id, meta_key, meta_value)
)
)/1048567, 2
)
AS `Size_in_MB`
-- Refer $table_prefix
FROM wp_postmeta
GROUP BY meta_key
ORDER BY `Size_in_MB` DESC;