Why build a large spaceship when a small one will do?

If I am aiming for a space victory, why should I build a large spaceship when I can build a small one instead? A small spaceship has two advantages: it takes less time and resources to build, and it is able to reach Alpha Centauri in less time.

This is a large spaceship:

Why should I build a large spaceship when the small spaceship below has a shorter travel time?