Textual uses the Webkit open standard to render the display of channel messages. This framework provides many advantages and disadvantages. It provides a very customizable experience for those who want to take Textual further, but on the other hand, it also uses a fair amount of memory for users that are in a large amount of channels for an extended period of time.
Textual is like a web browser, such as Safari or Google Chrome. Each channel that is open in Textual is the equivalent of an open website. The handling of each channel thus holds its own memory footprint. Removing channels that are not viewed often can have a drastic impact on the overall memory usage of Textual.
Changing the maximum number of messages shown in each channel can help memory usage beyond simply removing channels that are not frequented. This setting can be changed inside Textual's Preferences window. It is referred to as the "Scrollback Size" — reducing this setting to a lower number means that less messages have to be kept on the screen reducing what has to be kept in memory.
Additionally, although Textual itself and Mac OS X may report that the application is using X megabytes of memory this may not be true. There are very common instances in which Textual has already freed up the memory that it has been granted, but Mac OS X has yet to take that memory because no other applications needed it at the time.
If you believe there is a memory leak in Textual, then please report such a problem to #textual-dev on the Freenode IRC Network.