Every time a data or index page is read, it's first copied to the page buffer pool. Pages are allocated and deallocated based on the frequency of their usage, and tt's normal for the page buffer pool to completely allocated. If this particular database and set of tables are the most frequently used, then I would expect them to consume 50% or more. The reason why an entire table would be held in the page buffer cache is probably due to full table scans. SQL Server reads every page in a table or index during a scan, so the entire objects gets buffered to the cache, at least temporarily until it is gradually cycled out to make room for pages from other objects.