Bar width should surely be linked to the width of your shoulders? If yer a wee skinny runt, then 780 is gonna be wrong.
It's only dependent on shoulders when it comes to road bikes, something to do with the angle your hands interface with the bars and comfort over long distances.
With mtbs, it's purely a function of the length of your arm. I have long arms and legs compared to my body, so I run 780mm wide bars and a medium frame instead of a large as larges tend to have to long a top tube which is compounded by to having the front wheel further forward reducing my ability to load the contact patch effectively in a corner without assuming a poorly relaxed really aggressive position on the bike. Although I am going to try a 30mm stem now they are widely available on a large frame sometime.
I'm running tall bars as well these days, wide bars are good but the whole fashion with slammed bars is bollocks. The best positions in the middle of the bike, if you have a well fitting bike and you know how to ride you should be able to load up the front wheel with ease coming into a corner without having to resort to slamming your bar height. The latter of which is a lazy way of gaining cornering traction at the expense of the bikes playfulness.