This is a simple process, however not something I could find documented anywhere easy to find. MAMP supplies you with a range of sample configuration files:

/Applications/MAMP/Library/share/mysql/my-huge.cnf
/Applications/MAMP/Library/share/mysql/my-innodb-heavy-4G.cnf
/Applications/MAMP/Library/share/mysql/my-large.cnf
/Applications/MAMP/Library/share/mysql/my-medium.cnf
/Applications/MAMP/Library/share/mysql/my-small.cnf

To setup one of these for MAMP, simply issue the following command:

sudo cp /Applications/MAMP/Library/share/mysql/my-choicehere.cnf /Applications/MAMP/Library/my.cnf