I have this problem too - or rather, had - Rolo was XXXW, and I have very narrow hips

I hated riding him in a Barefoot, he had such a huge movement that I felt I was there on a wing and a prayer.
TBH, position was not an issue for us - I had no hope

- the only way I could sit comfortably on him was to have my stirrups short!

I looked like a Thelwell I tried a seat saver but I then felt too perched on top.
I rode my friend's huge mare in her GP saddle and her Isabell - and I found her much much wider in the Isabell - I could cope, but it was not as comfy as her GP (both saddles fitted the mare)
Bobby is reasonably wide, but the Vogue is so comfy, I don't notice (and he is not in the same league as Rolo

)
Have your tried Heather's irons? - they make it much much easier to wrap your leg around - your leg is not fighting the stirrup leather the whole time - they have made a huge difference for me - if I am able to relax, my feet just naturally fall in the right place

It all goes to pot as soon as I trot - but that is for another thread!

Hope you get it sorted - I think the stirrups may be worth a try
