Here is how it works:
Start MAMP PRO
Stop the server if it's running
Choose Menu -> File -> Edit Template -> PHP4 php.ini (If you want to change the memory limit for PHP5, then choose PHP5 php.ini)
The PHP ini file will be opened.
If a dialogue box pops up, read it and confirm with OK
Search (Command-F) for "memory_limit" (without quotes)
You should get the following line:
memory_limit = 8M ; Maximum amount of memory a script may consume (8M)
Now you can change the default value of 8M. For instance. into 32M or even more (like 128M)
Save (Command-S) your changes.
Close (Command-W) the file.
Start the Server.
Credits :
http://blog-en.mamp.info/2009/09/increase-php-memory-limit-with-mamp-pro.html