The last time I consulted a customer, he came up to me beaming with joy and said that he had taken my advice and changed all the primary key columns from BIGINT (8 bytes) to INT (4 bytes) and that had made a big difference! His MySQL 8.4 database is now 750 Gbyte smaller (from 5.5 Tbyte). Nice!
And yes, I know that contradicts the recommendations of some of my PostgreSQL colleagues (here and here). In the MySQL world, more emphasis is placed on such things (source):
Use the most efficient (smallest) data …
