Yeah, even with a large bouldering mat, it only covers a relatively small area.

(And there will inevitably be a moment when you look down and suddenly your large bouldering mat looks tiny.)

If you're going with a group of people, people will pool resources and lay out several boulder mats to cover the likely fall area if it looks like it could be awkward, and act as spotters for each other (to ensure that someone who falls does land on the mat).

If you're on your own, you need to think about where you're likely to fall (and if, for example, you're going to come swinging off at an angle), so you can decide where to place the mat.

There may also be problems where you decide that one mat won't be enough, or you don't feel comfortable without a spotter, and have to back off.

FWIW, I got a DMM Highball mat and it's served me well so far. I'm fairly comfortable going outdoor bouldering on my own with a mat. On my recent Peak trip, there were one or two things I backed off that I might have gone for if I'd had a spotter, but generally I felt comfortable judging the risks.

And the mat really makes a difference versus falling on the ground.