Starting with MariaDB 5.5.22 (more exactly: somewhere between r3337 and r3349 in the maria-5.5 tree) simple MEMORY tables start to consume more memory.

Example:

create table t1 (c1 int, index (c1)) engine memory;
– load 50 mio random rows
show table status;

Calculating bytes per row gives:

5.5.r3337: 16 (data) + 16 (index) = 32 bytes total
5.5.r3349: 17 (data) + 25 (index) = 42 bytes total