I think this is correct for CHAR and incorrect for VARCHAR.

A CHAR column always allocates the maximum size, so a utf8 (MySQL's weird "UTF-8, but only for the BMP" encoding) CHAR(100) needs to reserve 300 bytes per row, and a utf8mb4 (MySQL's actual UTF-8 encoding) CHAR(100) needs to reserve 400 bytes per row.

But a VARCHAR is dynamically allocated, so, yes, a VARCHAR(100) that stores 100 lower-ASCII characters is only going to use 102 bytes of storage (using 2 bytes for the string length).

See, e.g., the "tip" section at the end of https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8...