By default PHP has either 8MB (for PHP4) or 16MB (for PHP5) of memory on MAMP PRO (version 1.8.2). 16MB is certainly enough to run a basic install of ExpressionEngine but advanced image manipulation or use of the IP to Nation module will require more.
In MAMP PRO go to the File > Edit Template menu and then select the version of PHP you are working with. This will open the php.ini file in an editor window. Scroll down to find or search for ‘memory_limit’. This will be under the ‘Resource Limits’ heading on about line 230. It will look something like this (for PHP5):
memory_limit = 16M ; Maximum amount of memory a script may consume
To change the limit to 32MB we can edit it like this:
memory_limit = 32M ; Maximum amount of memory a script may consume
Select File > Save. Now you will have to restart the server for the change to take place.