I have no idea what is causing this pitting. It's uniform in pattern and my first thought would be the chain, but I'm sure I've only dropped it off the edge of the big ring once and this seems to be getting worse. Any ideas folks?

Shoe strap?

My shoes have laces – surely XT cranks aren't that soft?

measure the spacings between the pitting lines and compare with the spacings between chain link pins? Those marks look very uniform so this might identify/eliminate the chain as a cause.

Is it on both sides of the drive-side arm?

Nope, just that side.

Its from the chain dropping onto the crank on descents.
Mine after a week of the chain flapping about on alpine descents, now have a dual ring chain device to keep it in place.

in line with what twohats says, is that your lead foot when youre coasting? I'd expect the chain to drop onto or slap the top of the D/S arm as the lower run will hang looser, and if you predominantly lead with one foot then its logical to assume the 'top' side will get damaged.

I reckon the marks will measure the same as chain links and thats your problem.

What I also forgot to mention, was that most of the damage on my crank was caused by the chain coming off the bottom of the big ring whilst cross chaining on descents, and the rear mech pulling the chain so that it came to rest on the underside of the crank, causing the above without me never realising what was happening.
The above damage is on the underside of my lead crank so to speak.

Cheers for the replies – looks like that's what it is then

God aren't derailleur gears rubbish? When will we get our international standard centrally mounted gearbox from Shimano?

I'm sorry – isn't all of that just a sign that some folk are crap at setting up their bikes?

Well yes, maybe, but the question stands!

I'm sorry – isn't all of that just a sign that some folk are crap at setting up their bikes?

Nope.