It appears that large scale, in the context of transactional systems or websites, refers to both the large amount of data but also the large amount of interactivity and the complexity of maintaining security in such a big interactive system.


transactional websites, in the United States, seem to be banking institutions as per the following US government reference:

Transactional Websites

"Transactional websites provide customers with the ability to conduct transactions through the financial institution's website by initiating banking transactions or buying products and services. Banking transactions can range from something as basic as a retail account balance inquiry to a large business-to-business funds transfer. E-banking services, like those delivered through other delivery channels, are typically classified based on the type of customer they support. The following table lists some of the common retail and wholesale e-banking services offered by financial institutions.

Since transactional websites typically enable the electronic exchange of confidential customer information and the transfer of funds, services provided through these websites expose a financial institution to higher risk than basic informational websites. Wholesale e-banking systems typically expose financial institutions to the highest risk per transaction, since commercial transactions usually involve larger dollar amounts. In addition to the risk issues associated with informational websites, examiners reviewing transactional e-banking services should consider the following issues:
http://ithandbook.ffiec.gov/it-booklets/e-banking/introducti...

Large-scale Transactional systems, which are not banks, seems to refer to a similarly very big (in scale) and large in complexity and security requirements, as per the following US government website:

he 2003 DoD ABIS system, modeled after FBI IAFIS, was a single modality system (fingerprint only) with the goal of expanding to additional modalities. If successful, the DoD Next Generation ABIS, slated for initial operating capabilities the summer of 2008, would be the first large-scale, transactional production environment migrating from a fingerprint-only system to one with fingerprint, palm, face, and iris modalities. Many companies and federal agencies have adopted the ABIS concept; however, integration challenges remain a concern. As a result of 9/11 and even earlier border control requirements, AFIS systems are migrating toward “real time” response. When IAFIS was initially funded in the early 1990s, the response time for tenprint searches at the FBI was measured in months. With the advent of IAFIS operations in the late 1990s, the response for criminal searches has been measured in hours or minutes. With NGI, the goal is to provide responses in seconds for high-risk, National Security encounters to be searched against the KST and a few other high priority files
http://www.biometriccoe.gov/_doc/SABER-TechAssessment-Vol-1 ...