One of the words that makes me most cringe when I hear or see it in vendor marketing is the word scalable. It's no less annoying when buyers ask us "is EMC/FileNet/Hyland/Nuxeo/HP/Etc. scalable?"
Here's why.....ECM systems can be scalable or they can fail to scale well. They can have modular architectures that allow you to simply add more elements as required, rather than multiply the entire system as things expand. They can be scalable in that they have built in high availability, automatic failover support, run on enterprise grade application servers and databases. They can be scalable because they have been tested and proven to handle very high volumes (hundreds of millions of documents) in the repository and/or tested and proven to handle very high throughput rates (tens of thousands per hour or minute). There are many ways in which an ECM system can scale or not. But the biggest element determining whether the system can scale is your usage of it.
Consider that some users have many files (images/CAD files etc) that are a GB or larger in size, by contrast another may have an average file size in the small kb's (xml fragments for example). One user may handle a very small number of highly complex, large, ever-changing virtual documents, while another one a very large volume of static transactional images. Some firms want to centralize their ECM system and allow access to remote users via the web, others will distribute the architecture widely to combat latency issues.
In other words.....there are as many ways to scale an ECM system as there are to use an ECM system -- and no vendor out there has a monopoly on scalability. For where a SharePoint would be a good fit, a Documentum 6.5 may not, and vice-versa. Where an Alfresco may scale perfectly a better fit in another instance may be an Objective. Additionally it's always worth remembering that the ECM system is only as good as the operating system, database, application server, storage hardware, and the network that it runs on
The key of course as always is to fully understand your needs first, then match those needs against the capabilities of the products currently available, alongside your own architectural environment. And though it can be argued that some systems are more scalable than others, remember that every vendor will claim their product is scalable, you have to ask yourself "what does scalable mean to me?" And then test their claims accordingly.