Books
An elephant is a mouse built to government specifications.
— Robert Heinlein · Time Enough for Love

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
A joke about bureaucracy and over-engineering. Start with a mouse, then pile on committee demands and official requirements, and you end up with something enormous and clumsy: an elephant. The humour lies in treating an entire animal as the bloated result of specifications.
Context reviewed