The Flying Buttress
Wind blows against things. If it gets a chance, it can knock things down. The bigger something is, the broader and flatter it is, the more chance that wind will knock it down. Small wooden churches didn't have enough wall space for the wind to push against, so this wasn't a problem.
The first churches were made of wood. The thing was, you could make a building only so tall with wood — only as tall as a tree. Also, wooden buildings burned down a lot, especially in a time when fire was the main way to keep warm and cook food.
One good thing about wooden buildings was that you didn't have to worry about what would soon become the medieval church's second worse enemy: the wind.
One important thing that helped was the invention of the flying buttress. The buttress didn't really fly, but it had something like wings and looked like it wanted to fly.
What medieval builders did was to take the buttresses that normally pressed against the walls and move them out away from the walls. They connected buttresses and walls by a long arm or wing of stone. The thrust of the vaulted ceilings still came down the piers and walls, but also moved down the arms of the flying buttresses, down the buttresses themselves, and into the ground. The flying buttresses helped spread the weight of the vaults over more supporting stone. That meant the walls could be thinner and still hold up just as much weight. The piers could be thinner, too.
With the walls thinner, windows could be much larger. With the piers thinner and the buttresses farther out, more light could come through those larger windows.
The windows got to be so large in gothic cathedrals that the walls seemed made of glass.
As medieval people started building in stone, they also started building taller churches. Taller churches meant more wall space for the wind to push against. They could only build so tall or their walls would get knocked down.
Another problem kept down the size of a medieval church. When the people began using stone in their walls, they used it for their ceilings, too. They formed the ceilings out of curved surfaces of stone held together by a layer of concrete. They called these new stone ceilings vaults. A stone vault is very heavy, much heavier than a ceiling of wood. That weight became the worst enemy of medieval church builders. The vault pressed down against the stone walls. The walls could be only so high before the force of the wind and the force of the heavy vaults made them collapse.
The answer to this problem was the buttress. A buttress is basically a huge stone prop that pushes in against the walls with the same force that the vaults push out. The red arrows show the force with which the vaults push against the piers and the buttresses. Using buttresses against the outside walls, medieval people could build taller churches. To help the buttresses, they also made the walls very thick. The piers that held up the vaults on the inside of the church also became very thick.
All this helped to make taller churches, but it also made for dark, heavy buildings. Because the walls were thick, windows were small and there weren't many of them. And the thick piers and buttresses blocked the light. The early stone churches were gloomy inside.