The bestselling "adventure" bike in the world is, absurdly, BMW's R1200GS.  I'll grant you: It's comfortable, reasonably robust and divine on the road (100hp+ from a low-mounted boxer doesn't hurt). But bear in mind the compromises inherent to motorcycling: There's no such thing as a delicious low-carb lunch, and this beast is a fat slice of cookie dough cheesecake. It weighs over 500lbs and costs $16,000 and up. BMW also does an "Adventure" version that comes with some accessories like a bigger tank, a heftier price and correspondingly increased pudge.

BMW R1200GSA

That's not a motorcycle, that's a Civic with a side stand. Why the heck does anyone buy BMW's cash cow to go off-road?

3 reasons, as far as I can figure:

1. BMW's marketing tries their very best to make the GS look like a dirtbike with a windscreen. ("Travel enduro" that has an "enduro pro" electronic configuration - give me a break)

2. GSs starred in Long Way Round, a TV documentary series with Ewan McGregor and Charley Boorman, where they rode around the world (and then to the southern tip of Africa in the follow-up, Long Way Down)

3. Partially as a result of 1 and 2, there is a widely held but naive belief that a two-wheeled machine weighing 500lbs will make it through even mildly gnarly terrain without extreme physical exertion on the part of the rider.

See, what I really needed here was another 200lbs and 90hp to yoink out of the mud 

Following the GS's surge in popularity in the mid-2000s (McGregor and Boorman likely had something to do with that), other companies were quick to jump on the high-margin bandwagon. The R1200GS now has esteemed company in the 1000cc+ furniture class. For example:

The stupefyingly overweight but charismatic Triumph Explorer 1200 -

Triumph-Tiger-1200-HD-Wallpapers0

Ducati's herniatingly fast and seductively sexy sportbike on stilts, the Multistrada 1200:

Multistrada

Yamaha's bonkers Super Ténéré (which used to mean "desert" or "wilderness" but now means "where is the nearest paved road please")

2015-Yamaha-XT1200Z-Super-Tenere-EU-Matt-Grey-Action-001

The quirky and unashamedly road-hugging Moto Guzzi Stelvio:

Stelvio

And the biggest pants-on-fire pretender of them all (albeit the most convincingly rugged), the KTM 1190 Adventure:

2013-ktm-1190-adventure-r-action-13

Riding a 500lbs+ bike off-road is possible (and highly marketable) - it just takes serious skill and stamina.

http://www.youtube.com/watch?v=NpCubYprqFQ

So what's the problem with all these monstrositycycles?

Even if you're Chris Birch and you manage to keep the rubber side down most of the time, the occasional tip-over during your journey through the swamps could present a trip-ending catastrophe, leaving you and your Bavarian sofa stranded in three feet of mud. A rider on a lighter bike would simply pick up, winch out if necessary and move on.

Say you're a bodybuilder and you have no problem setting a 500lbs bike upright on a sloppy muddy slope, ten times in a day - problem solved? No. When that much mass falls over, it's storing kinetic and potential energy that gets transferred back to the ground, sometimes via the bike's hard parts (levers, shrouds, gas tank, handlebars - which then break) and sometimes via your frail body (which also breaks). Trust me when I say you'd much rather have a slim 300lb bike fall on you instead of a bulbous 500lb one. (if you don't trust me, trust Walter Colebatch, who knows more about ADV riding than any of us)

So - what are the alternatives? I'll get to that as part of another post, but in the meantime, keep this rule of thumb in mind: Never ride a bike a bike off-road if you can't pick it up. On the contrary - if you're riding ADV, ride the bike that would be your pick on the worst section of your trip.