Saddles usual slip back for one of three main reasons
(i) the tree's too narrow, so the saddle slides back to a narrower part of the back
(ii) the saddle tree's too wide, so the front naturally drops and slides backwards
(iii) the saddle tree's the correct width but the panel shape is wrong or needs adjustment
Your horse looks slightly croup-high so (ii) is less likely, and being a larger rider is only likely to affect a flocked saddle. They're often very soft nowadays and a heavier rider, with balance biased forward due to the horse's conformation, is likely to compress the front flock more quickly than someone lighter.
And of course I'm talking English saddles, though I suspect the same would be true for western. I've little experience of the latter so don't usually try to advise - there'll be someone with far more experience of them somewhere on here
