Hard to say...
I have the 4" paring knife. I found it some years back down in Oregon. It was just a little rusted thang, but it cleaned up nicely and I kept it for a patch knife.
It is ground on one side only, which makes it ideal as a patch knife.
My father in law has the 7" butchers knife and I think it is quite a bit heavier but I have never compared 'em side by side.
Now, my father in laws blade is rather old, I understand the new knives are thinner? Not sure.
This page - http://www.ragweedforge.com/HistoricalKnifeCatalog.html
Seems to indicate that they are about the same thickness, but I'm not sure how accurate that is.
I recommend you simply email Ontario and maybe even Ragnar at Ragweed forge. Hes always been helpful to me in the past.
Modifying the Ontario ( Old Hickory ) butcher knives has quite a following -
I can't imagine this would so popular if the smaller Old Hickory blades were as thick.