See figure 11.7 and 11.8 in the book. The view on the left is better. In that view the dam is under compression. Most materials that dams are made of are stronger in compression. Dirt has almost no strength under tension.
Concrete is much stronger under compression than tension.
Steel does have strength under tension, but is still stronger under compression. Thus one should build the dam so that it is under compression due to the force of the water, not under tension. The larger bridge is more likely to collapse under its own weight. When the bridge is scaled up by a factor of two, the bridge becomes 8 times as heavy since weight scales as volume which scales as the cube of the length multiplier. Strength scales as the cross-sectional area which scales as the square of the length multiplier or 4 times as strong.