Bull gator question

I got a stand in my lease that is on a tram road. The tram dead ends at a cypress pond, and my stand and feeder is on the edge of it. Saturday morning I eased down there real easy on my 4 wheeler and shut it off when I got to my feeder. Soon as I shut it off, a bull gator started bellowing out there in the pond. It sounded close and loud enough I was surprised I couldn't see it. I mean it was chest rattling deep and loud. No question about what it was. I guess he thought the sound of my 4 wheeler was a rival alligator.

Question is..how big of a gator does it take to make a deep bellow like that. I tried googling it but all I came up with was gator football spam/garbage/crap :shifty