Thus ice is less dense than water. A kilogram of aluminum has more volume. Since it is less dense, the same mass must take up more space. A liter of water is heavier than a liter of ice because the water is more dense than the ice. Thus the same volume of water has more mass than that volume of ice.
Weight is proportional to mass.
The top of the spring has to hold up the whole spring while lower down the spring is only holding up part of its weight. Thus there is more force on the top of the spring and it stretches farther. 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.