They are extremely large because they need to be. Due to the amount of content, they are many levels deep with intricate UI. Many times they are made up of many micro-websites and subsections to break up information and content as well as serve multiple entry points. They provide better control over SEO with multiple pages and changing content.
Multi-page sites makes a lot of sense for users who need traditional navigation to get around. They work very well for giving users a visual map of where to go.
For instance, for e-commerce sites, users don’t want to spend a page learning about your company’s story or staff. They want to find the right product, pay you, and move on with their lives. In that case, a nested navigation leading to different pages can be quite effective.
Multi-page sites also give you a better chance to rank for different keywords, since you can optimize for one keyword per page.
In fact, the hybrid single-page site (like Hell’o Baby above) emerged to give designers the best of both worlds. The single-page structure optimizes conversions. On the other hand, the top navigation gives users more control than a pure scrolling experience. And as designer Cameron Chapman suggests, you can even use modals and AJAX techniques to support multiple pages of content.
The deeper the website goes, the more a traditional navigation will struggle.
Some people believe that navigation can comfortably accommodate the first three levels but beyond that, expect one of two possible outcomes:
Either the navigation has to expand, eventually taking up more screen real estate Higher pages in the information architecture will no longer appear in the navigation. In the latter case, if the user is deep within the website, they will lose the context of where they are because they are not seeing where the current page fits in the website’s structure. As the site expands, we need to consider the experiences produced by all the different entry points.
If there are multiple-entry points for the website, the user must be able to easily navigate back to the home page or other pages of interest.