Doing small software iterations, frequent releases, a little bit at a time, could go one of two ways. Scope out too much and to keep the release on schedule skimp on the design and implementation, or even worse - quality assurance.
- or -
Scope out just the right amount and put the full press into design, implementation and quality assurance.
Both could equal a couple of weeks work, but one is a building block, and the other a throw away.