Why is it called a minigun when it's clearly not a "mini"gun?

Compared to the 20 mm Vulcan air craft cannon it was based on, it was a "mini".

The first motor driven gatling gun, the Vulcan 20 mm cannon is huge and takes up most of the front of a fighter jet.

The mini gun is 7.62mm and much, much, smaller.

A Minigun

You are right. It is a misnomer. What it means is that it is a 'mini-cannon.' The Nazis referred to their 20mm and 30mm automatics as 'automatic cannons,' and so the term evolved to 'mini gun.' If you think about it the term mini cannon would be more accurate.

H

Wel, when compared to other military machine guns it is rather small....even though the gun has 8 barrels they are pretty short....and the gun is rather compact....for a vehicle mounted gun..

In military parlance, a "gun" is a mounted weapon. Rifles are rifles, and pistols are sidearms. Guns are big, heavy and mounted to something.

When they say "mini", they mean the cars.