Most dies today are dimensioned towards the minimum side of the spectrum. Sizer dies (in my opinion) size brass down more than needed. Expander dies tend to favor the smallest diameter bullets for a given range for any particular caliber. Larger bullets are accomodated by simply using (again, in my opinion) an excessive amount of belling.
If you mic the shaft of the expander plug, I wouldn't be surprised to hear that it's about .354".
The priority of die makers is to ensure that you end up with minimum dimensioned ammo purely for the sake of functioning.
They don't care if the results are compromised accuracy and shortened case life. For better or worse, they realize that most reloaders are just looking for cheap ammo that goes "bang".
My personal experience has been that special attention needs to be given to die dimensions. If you want accuracy, this is just one more area that needs to be considered. I bought a set of 44 Special dies from Redding. They were ridiculously undersized. A sized case looked like a 44-40. The inside neck dimensions were .424". The expander plug wasn't much larger. Loaded ammo was just about worthless. My carefully cast lead bullets (.432" diameter to match cylinder throats) were squashed in diameter, and the bases were visibly distorted. I called Redding and they told me the dies were within spec. They didn't care less about my concerns.
So, I made my own brass laps and used lapping compound to open up the sizer die. Eventually, I got it so sized cases had a .429-.430" I.D. I used an old Lyman M die with a stepped expander so I could press the bullets partially into a case. The resulting ammo looked better and certainly shot better. (For someone who just loads .429" jacketed bullets, my modified dies would be a problem.)
The same thing has happened for me with every revolver cartridge I've tried from 38 spl to 455 Eley. (Lee makes one single 45 sizer die that they sneakingly package for 45 ACP, 45 Colt, and 455 Webley.)
It's too bad the die makers don't pay closer attention to this. They should be specifying die dimensions in writing so customers can make informed purchases.
Conversely, most gun makers cut their chambers on the larger end of the spectrum. Again, the idea is to accomodate the "I just want it to go bang!" crowd.