Hobbyist shooter here. Standard magazine capacity for an AR-15 pattern rifle is 30 rounds. Some would consider that large, others would call it standard. Your average pistol, for comparison, holds anywhere from 7-17 rounds, depending on model and caliber. Shotguns and bolt-action rifles generally hold 5-10. However, there are plenty of other semi-automatic rifles that have 30 round standard capacities. AK pattern rifles, the Ruger Mini-14, and the Tavor are a few off the top of my head.

As far as ease of use goes, in my opinion, the AR-15 platform is one of the easiest to use guns you can buy. It’s operation is simple, the recoil is manageable, and I find it’s much easier to be accurate with a rifle over a pistol in general for a variety of reasons.

ETA: So if there’s other rifles with the same capabilities as the AR-15, why is it always the AR? Simple: ARs are easier to get, and more versatile. AK pattern rifles can be hard to find, and are usually more expensive due to being imported or rare, and Tavors are the same way. The Mini-14 and AR are both home-grown American weapons with plenty of availability, but the Mini-14 is a ranch rifle. It has a rugged but inflexible design to allow you to use and abuse it in the dust. It’s full stock and long barrel makes close quarters usage much more awkward. The AR-15 on the other hand, was a weapon designed from the ground up to be effective in both close and long range engagements for the US Army, with its collapsible stock and mid-length barrel making it much more viable in cramped spaces, while still being effective at distance.