Why not rebuild a self-repairing cruiser into a battleship?

What prevents a self-repairing Glorious Heritage cruiser from building itself up into a battleship?

The size of the slipstream drive is the limiting design constraint.

Self-repair lets the cruiser replace components and manufacture material from asteroids. It does not let the ship change the scale of the slipstream drive around which its architecture was built. A battleship would require a larger propulsion system and the internal arrangements to support it. The limiting factor is fundamental hull design rather than access to raw material.