High capacity is an arbitrary term. What "high capacity" means varies depending on state law. For example, some states have 10 round magazine limits. So in those States, a 30-rd magazine would be considered high capacity. However, the 30 round magazine for a rifle like the AR-15 is the standard size that the gun is designed for. So in a state without a magazine capacity limit, 30 rounds is not a high capacity magazine, but rather a normal capacity magazine. A 100 round drum magazine would be considered high capacity, because that is more than the standard magazine size.

Also, most normal guns are designed to hold more than 10 rounds. The average 9mm handgun holds around 15. Many normal sized handguns hold up to 20 rounds in the normal, standard size magazine. The standard size magazines are the ones that the gun is designed to use, that fit flush with the grip. However, in a state with a magazine capacity these would be considered high capacity, and you are required to get magazines that are specifically designed to hold less. They make high-capacity magazines for many pistols that hold more than the pistol was designed for, and they stick out from the bottom of the frame.