The Jargon File (version 4.4.7, 29 Dec 2003):

second-system effect
 n.

    (sometimes, more euphoniously, second-system syndrome) When one is
    designing the successor to a relatively small, elegant, and successful
    system, there is a tendency to become grandiose in one's success and design
    an elephantine feature-laden monstrosity. The term was first used by Fred
    Brooks in his classic The Mythical Man-Month: Essays on Software
    Engineering (Addison-Wesley, 1975; ISBN 0-201-00650-2). It described the
    jump from a set of nice, simple operating systems on the IBM 70xx series to
    OS/360 on the 360 series. A similar effect can also happen in an evolving
    system; see Brooks's Law, creeping elegance, creeping featurism. See
    also Multics, OS/2, X, software bloat.

    This version of the jargon lexicon has been described (with altogether too
    much truth for comfort) as an example of second-system effect run amok on
    jargon-1....


The Free On-line Dictionary of Computing (30 December 2018):

second-system effect

   (Sometimes, more euphoniously, "second-system syndrome") When
   one is designing the successor to a relatively small, elegant,
   and successful system, there is a tendency to become grandiose
   in one's success and design an elephantine feature-laden
   monstrosity.  The term was first used by Fred Brooks in his
   classic "The Mythical Man-Month.  It described the jump from
   a set of nice, simple operating systems on the IBM 70xx
   series to OS/360 on the 360 series.  A similar effect can
   also happen in an evolving system; see Brooks's Law,
   creeping elegance, creeping featurism.  See also
   Multics, OS/2, X, software bloat.

   [Jargon File]