The 865 engine was the largest production vertical twin made until 2009 when the thunderbird came out (that sounds a bit gay) it is not a little engine...motorcycle manufactures have learnt that there are customers (and there are lots of them) out there that care only about how big it is.
It may not do anything better than a smaller one but a lot of blokes just have to tell there mates...hey I've got a big one.
