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.