If you weigh 180, and you put 180 in the bar, each leg is approximately lifting half of the total weight i.e. 180. If you did a single leg squat with no weight on your back, a single leg is lifting about 180.
It's sometimes true that single leg squats are easier, because though they require more stability, they don't require you to also support a heavy weight on your shoulders. There may also be truth to the idea that bodyweight exercises are more 'natural' for the body.