I was clicking around that site again (www.mercedes-benz.com/e/default.htm), and by comparing the three engine options (500 and 320 gas, and 400 diesel) in two body sizes (Station/Long and Station/Short), it seems that the body size is one of two towing capacity factors:
All three engine options (even the small 320) are rated 7,700 lbs in the long wheel-base versions; and all three engine options (including the high torque 400-diesel and high powered 500) are rated only 6,283 lbs in the short wheelbase body.

I guess it's only a question of having enough mass (inertia) to properly control the trailer, rather than enough power to pull (which all the engines, even the little 320 gas) apparently have. Which doesn't say much for the M-class, which is rated (at least here in the USA) at only 5,000 lbs whether you have the 320 or the 430 gas engines.
But the second factor will surprise you even more (next message):

However, something else that I discovered relates to what I noted in the first messsage. Here in the USA, all the M-class brochures, for both the 320 and the 430, list 5,000 lbs as their towing limit. Yet at the corporate website, it lists 7,418 lbs for both engine models. [in all cases, trailer brakes are assumed]

Apparently, MB derates towing capacities for the USA, so far for the M-class, by as much as one-third!

[But why? Could it be this country's penchant for lawsuits? It has long been rumored that MB withheld ABS in USA cars for a decade because they waited until they were convinced that it presented no liability questions -- i.e., it was adequately trustworthy.]
In any case, this derating is disturbing. It can well be that the GWagen's towing capacity will be similarly derated (from fear of litigation?) from our coveted 7700 lbs to a mere 5,200 lbs. This would be bad, not only from the standpoint of warranty coverage, but also legal liability and insurance coverage in the event of an accident, if we violated this USA decree in favor of the international ratings.

Let's hope that they don't derate the G -- we'll have to wait and see the official USA brochures to find out!

Looking at the hardware, I think what limits towing capacity is the hitch itself, which is only rated for 5,000 lbs. Also the rear springs sag quite a bit at the limit. Do Euro models use a different hitch, and/or stiffer rear springs?

__________________
Lenny

There are only three sports: bullfighting, motor racing and mountaineering; all the rest are merely games. --Ernest Hemingway

'10 GL550/'04 BMW 545/'99 BMW 323/'98 ML320/'87 VW GTI (race)