I can't of course speak for every gun ever built, but all that I have ever checked have had a slight increase in wall thickness as the choke was approached. Thus I have never found the thinnest point of the bbl wall to be at the juncture of the bore to the choke cone, but a few inches back. This would definitely seem to negate it being a simple change of powder.
Whatever the ultimate cause it surely seems the shot charge is being "Checked" by the choke & thus having the effect of an obstruction. This would account for the "Bulge" behind the chokes as the check would occur on the front of the shot column hitting the choke.
They haven't started proofing with a bore size ball have they??

Miller/TN
I Didn't Say Everything I Said, Yogi Berra