I had replied to a similar question on another website, so I will cut and paste the original quote and my reply. It addresses some of your questions.
QUOTE "Keep hearing similar things from people who say that taking a "small" bike on tour is not a good idea. It doesn't have the power, ride stability, and round-the-clock saddle comfort of a bigger bike. Been skeptical of that, especially when it comes to the f650. Inspiring to read about all the long-haul adventures on the site." END QUOTE
I have been on the road for 18 months with 2 years to go, and 95% of the bikes out here (currently south america) are 750cc or less. Most of us out here doing RTW or longer trips in general are quite budget minded, that means that fuel economy is very important, but the other part of that if we are not camping, we stay in cheaper hostels or hotels, often without a garage. The bikes are always parked inside the hotel lobby or courtyard. Manouvering a 1150ADV up a curb up two steps, down a hallway into the living room of the family that owns the place would not be inviting night after night. The other fiscal componant is when travelling in Africa and many other coutries, a carnet is required, which is a type of passport for the bike that assures the country that you are going into that you will not sell it on the blackmarket and skip away with the money. This means a cash deposit (or line of credit assurance) worth (depending on the countries that you are going to) up to 150% of the value of the vehicle before you leave on your trip. So in my case, if I took at USD$18000 1150GS, I would have to leave $27,000 as a deposit on the bike since I don`t use a LOC. (You get the money back once the bike is back in the country of trip origin, but no interest.)
Having said that, even if our budget was unlimited, my choice would still be a "small bike". Bolivia, where we are now, has something like 5% paved roads, and 100% of them are sprinkled with dogs, ducks, potholes, cows, missing lanes, people, horses and buggies, and loony bus drivers. From a common sence safety perspective, going fast just isn´t smart here. So why would one need all the extra power? And for ride stability, it has been my observation that one of the contributing factors to that is the big bikes haul too much shite! Its a downward spiral: big bike means you can carry lots of luggage, and since you have lots of luggage, you are convinced you need a big bike. Same old More is Better western thinking drone.
And for the final "round-the-clock saddle comfort", if one is riding around the clock and not stopping to see anything, then they are a tourist, not a traveller, and would be best advised to hang out with the nice folks at Iron Butt. They like big bikes, too.
I have toured on a 1150GS to Alaska and loved it, and if one wanted to do a Big Cities of the World tour, connected all by tarmac, then ´giddy-up and happy trails. But there are so many remarkable places with incomparable people and hospitality that live far away from the cities and the chaos of all the "benefits" of being connected by pavement, that if you really want to experience a life away from most of us, get to another country, find some dirt roads and go say Hi.
And if I may, I would suggest you hurry, as this place is getting smaller and smaller all the time.