A friend ran into an issue with their Wordpress database becoming huge. First step was to figure out which table(s) was causing the issue. Doing an online search came up with this handy little MySQL query which returns the size in MB for all the tables in a database:
SELECT table_name AS "Table",
ROUND(((data_length + index_length) / 1024 / 1024), 2) AS "Size (MB)"
WHERE table_schema = "database_name"
ORDER BY (data_length + index_length) DESC;