It fits well. I'm glad that I got the Large. The specs on the Medium imply that it should fit the XG750 but I think it might be a tight fit, and I think it's good to have some air in there and makes it easier to put on and take off. It covers my bike completely to the floor. I keep my bike in a garage so the cover is mostly to keep it clean. It does get damp in my garage during the winter though. I get rust on my brake rotors when my car and bike sit during the winter. My garage is not heated.
I didn't notice any interior heat insulation. I wouldn't put a cover on a vehicle while it's hot though. Also, these bikes vent fuel fumes when they are hot. There's a rubber vent hose that sticks out the bottom. The description says "We’ve also UV treated this cover to ensure maximum sun and heat protection." So, there is heat protection from the sun.
The Platinum Tyvek cover says it is "best" for Water & Rain Protection and Snow & Ice Protection, whereas this one says it is "better" in those 2 areas, so if you are going to store your bike outside, especially in winter, that might be a concern. Otherwise the 2 covers are comparably the same, except that the Platinum Tyvek cover is 2 layers thick and this one is 3 layers. Both are made in the USA. All of their other covers are 1 layer and not made in the USA.
I think it's safe to say that the large size would fit over saddlebags and the crash guards and pegs. I'm not sure about the sissy bar or the windshield. The bike in the picture looks like it has a sissy bar. You should be able to return or exchange it if it doesn't fit.