Why do British military honor guards have such big hats?

That's a bearskin hat. Members of the Royal Guard wore bearskin hats and red uniforms. The Royal Guard has been wearing bearskin hats for about 200 years. This hat symbolizes the great military power of Britain.

In fact, French soldiers were the first people to wear this kind of hat. At that time, the French army wore this bearskin hat to make soldiers look taller and more deterrent. However, after defeating Napoleon's army in the Battle of Waterloo, British soldiers began to wear bear fur hats instead of 18 15 to show that their combat capability was better than Napoleon's army.