When the structure of a large implementation is altered, it can create severe ranking problems, especially if the link pathways need to be disrupted. For this reason, whenever changes to the website structure occur, rank preservation comes front and center, as one of the primary concerns.
The premise is the same for both small and large sites. The main difference is the role played by automation and other consequences of scale. On a commerce site, the change could be initiated by a new product line. This often requires the creation of new pages and the removal of old ones.
But what happens to the old ranks held by the old products? Rank preservation is often handled by 301 permanent redirects, pushing the visitors from the old pages to the new ones, and passing rank along as well. The concept is simple, but how do you implement a thousand page changes all at once? Small sites often use meta redirects, page by page. While that may sometimes be useful to a large enterprise, handling large numbers of redirects is better handled by robust data management, where the redirection occurs automatically because it's built into the rules set.
When you purchase a successful business and the accompanying web implementation, how do you integrate the new with the old and preserve or improve the most productive ranks? Again, this is a problem unlikely to be encountered by the small site, but is very frequently a consequence of standard business strategies of large enterprises. And whether the enterprise keeps all sites live, or integrates them all into one, enterprise website, the notion of rank preservation is central to good web planning.