Para,
Not so quick. Then, as now, there has always been a wide variation of knives lumped under the Bowie designation. The butcher knives were a little thinner maybe 1/8 - 3/16", but actual survival / sidearm / sporting bowies were heavier and sometimes much heavier. I have seen plenty of examples with blade thicknesses of about 1/4", and some have been spoted with 5/16 or heavier blades.
You might be confusing the thickness of the guard, which was usually thin with the blade thickness. The big thick 1/4" plus guards, with heavy lugs etc., are a 20th century invention.
N2S