About midway between the chest fastening and the top seam at point of the wither, make a dart on each side - my mum used to do this for my immense-withered old TB, she would make a kind of pleat and stitch it into a long, thin triangle to shorten the neck bit - seemed to work well (sorry about untechnical sewing terminology!)
It may be worth keping an eye on ebay for half neck fleeces as well. I had the same issue with my tb and picked two up quite cheaply.. the problem is that you dont see them very often.
If it's long enough along the back, i'd move 1 front fastening so there's an overlap, to keep it more forward - if i've understood the problem correctly this should help.
All good advice here - and I find the ones with a substantial satiny lining on the front help too - cheap ones without this seem to stretch more and work their way back. The weatherbeeta ones with the big satin front sections seem good.
I have this problem, it is a nightmare for rubbing his shoulders bare. It doesn't happen with stable or turnout rug but any fleece seems to do it even the wetherbeeta one just mentioned.
I would use find something else to use, if you are using this as an under rug, I would change to a proper one like the Rambo Duo liners. I don't think fleece rugs make good under rugs at all, they stretch and rub when used under another rug. They are ok for short periods of use, eg while the horse is cooling/drying off. I hate them, will never own one myself.