system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why are some Water Towers very tall and skinny with small reservoirs while some are pretty short and fat with huge reservoirs.

I know it has to do with water pressure and gravity but the range of size and height, which I have seen, is quite vast.    In fact, by our home we have 2 that are on either side of the freeway.  One is tall and skinny with a small reservoir and one that is short and wide with large reservoir.  Why?
gpt
One major factor for the height of the water tower depends on the terrain.  You need to have the tank high enough over the area supplies water to for enough pressure.  If the ground is flat you need to build a tall tower but if there is a hill that is high enough you can just put the tank on the ground on top of the hill.   

The size of the tank will depend on the usage of the network you are supplying, If you design the tank so it can provide enough water for 24 hours even if there is no power it is quite obvious that the size depends on demand.

You can build the tank so it lasts longer or so it is large enough to if the city grows. It is also a question of money so tanks you place on the ground can have extra capacity for a minimal extra cost.