All else being equal, chain width is irrelevant to strength. Check with the chain manufacturers. For example, KMC rates both their Z510HX (1/8") and Z610HX (3/32") chains to 1,200 kg. These two chains are identical in every way except width. Other KMC chains are rated differently because of their different materials and design, not their width. I'm no engineer, but I suspect that sideplate x-sectional area would probably be the most important factor, and these can be the same for 1/8 and 3/32.

from the KMC website..."Z510HX & Z610HX: This is a perfect chain for racing and Heavy Duty BMX use, the tensile strength is a good 1,200 Kg. It
can be used on almost every front sprocket and rear freewheel/Cog. Light but strong, this chain is a
bestseller in the BMX Freestyle world, but it is also becoming very popular in the mid-high end range
internal gear hub equipped bikes, because of it's long life and high reliability."

http://www.kmcchain.com/index.php?ln=en&fn=bu_bicycle#c