Edit: I’m referring more to those heavy cruising bikes than specifically Harleys, but it seems the engine traits are similar enough anyway.
So from what I’m getting from the answers, the reason heavy cruising bikes sound so deep compared to dirtbikes is primarily the resonance due to engine size as well as being a four-stroke design. And since the extra tinny-sounding two-strokes aren’t much of a thing anymore, it’s mostly due to engine size now. (With some influence from muffler design.)
Thanks for the answers!