I feel it's a combination of both weight and balance. Too heavy, my old Tamron 200-500mm for example, and "the shakes" set in too early to be practical when holding it up to my eye. The focus control is too far down the lens.
Unbalanced, my Samyang 800mm mirror for example, and I find it very difficult to hold the silly thing still. There's nowhere to put my left hand that doesn't risk knocking it out of focus.
Well balanced, my Sigma 150-500mm, I can brace my left elbow against my chest and hold it by the tripod mount. Any fine focus, a/f over-ride when shooting through foliage, for example, can be achieved with my thumb.
I'm not sure about extra stability when adding a battery grip, for example. I feel any improvement here may be simply due to the better grip one can achieve on the bigger assembly.
YMMV
Last edited by kypfer; 04-25-2019 at 10:25 PM.
Reason: spelling