We tend to build rifles with much larger rods than many of the originals. I think that is to compensate for the tighter loads we tend use in comparison to what was originally used.
I can't speak to the difference in size between modern and period practice, but if this is so then another possible reason might be that modern folks are used to thinking about wood strength in terms of modern sawn dowels rather than split or coppiced rods.
Of course, when replicating older technology it seems the we tend to overbuild
everything except perhaps the barrel and lock. It isn't just guns, either, but swords, knives, tomahawks...
