Sound has a unique ability to tap into our emotional psyche. A moan, in particular, can convey a range of emotions - from pain and distress to pleasure and ecstasy. When we hear a moan that is "getting bigger," it implies a crescendo of emotion, a build-up of intensity that is hard to ignore.
Without more information or context about the video, I can't provide a more detailed or accurate summary. If you're looking for information on a specific video, I'd be happy to try and help you find it.