This was the toilet in a house we are bringing up to date. It was the usual set up toilet with no sink. We are moving the toilet so we can have a corner sink as well. Hal asked a question about whether a vent pipe would work if it was not 100m, this one which is on mains drainage had an 80m pipe going into 100 in the garden. The toilet still flushed with no gurgling, it had 100m exit reduced to 80 to go through the floor then back to 100. Clearly hal is not alone in wondering does size matter
Next week it will be finished and should look better.
Its a sensible precaution having the initial pipe at 80mm especially if the évacuation might have other WC's or sinks etc upstream of where this one joins in.
The throat of a WC pan is less than 80mm in one axis, that is intended to catch anything that might obstruct further downstream, running an 80mm pipe from there without any bends is OK before joining in to the 100mm.
Where I have 6 wcs on 3 floors going into a common downpipe I use 100 mm from the WC to the 125mm soilstack, one one floor where I had to run 2 into one soil stack connection I used 80mm on one joining into 100mm from the other to the 125mm stack.
Using "throat" as a measurement implies the bore, look down your dunny, does the bore look like 100mm? No it wont.
What I was trying to get across and seemingly failed in your case was the wisdom of progressively increasing the size to make sure that any blockages remain local to the WC where said obstruction was introduced and not back up the rest of the sanitary ware and that an 80mm straight section of pipe from a WC before any bends at 100mm will not be a problem and is a good idea.