From LQWiki

To scale is to expand or grow with an organism that itself grows. This term is often used to refer to enterprise technologies that are built from the ground up with "scalability" in mind, i.e. they can be used for a minor operation, but if that minor operation grows to a major operation, the technology will grow with the expansion of the operation.

An example is the program dot that can draw a mindmap out of a text file. It was created with mindmaps in mind and works fine for 700 edges. But if you want to use it to create a mindmap for the 30000 edges and 4000 nodes that http://wiki.linuxquestions.org provides, it needs more than 20 GB of RAM and cannot be used. This is called a scaling issue.