At present, there are four (possibly five) main databases in the business market; Oracle, SQL Server, Sybase, DB2 and MySQL. There are a great many other RDBMS systems out there, but these tend either to be legacy databases or used within academia such as universities or further education colleges. A physical data model on each implementation would be significantly different, not least due to the underlying OS requirements that sit underneath them. Examples would be SQL Server which only run on Microsoft Windows operating systems, while Oracle and MySQL can run on Solaris, Linux and other UNIX-based operating systems.
This means that the disk requirements, security requirements and many other aspects of a physical data model will be influenced entirely by the RDBMS that a database administrator (or his organization) chooses to use.
Whilst there is increasingly debate surrounding which RDBMS is better within various domains, it is generally accepted that Oracle's architecture is best suited to enterprise & larger implementations, SQL Server better for SME's and MySQL adequate for SME's and small businesses. A useful resource for such debate (which contains useful case studies) can be found at the IT QUEST web site.