In general, modern guide diameters are moving down.
I think braid has a lot to do with it, but this expensive 7'2" glass St. Croix Legend MM, made for 3000-size reels, has the first K-guide not much bigger (20 mm) than on the Yamaga blanks UL I posted first. The smallest running guides on this rod match the intermediates on the UL above.
Four K-guides to tiny, 5 small running guides.
It casts 12-lb fluoro very well, including lure weights below the rod min rating.
And I remember my first inshore rod, Berkley TriSport, giant stainless loop guides. The first two guides were both larger diameter than the spool on the Mitchell 300 I matched with it.
It would cast to mackerel off the jetties with 17-lb Stren.
The 2-pc glass 7' TriSport with metal spigot was more than twice the weight of the S-glass St. Croix just above.
They actually cast very much alike, as I can remember that far, and have to give some of the credit for the weight difference to the design of modern spinning guides.
Here's the older idea, where the first loop guide is about the same diameter as the spool.
I don't know that this makes a difference. I've seen my buddy fish with his spinning reel on a Lami casting rod because that was the long rod he had with him - with casting rod guides - the combo cast well-enough, it made me question any cone of flight theory - the line isn't pushing through the guides, it's being pulled through.
I measured one more. My 7' offshore E-glass, made for up to 6000-size reel, has Fuji HNOG Concept guide set, with 6 k-guides + tip-top. .
24 mm, 12 mm, 10 mm, and the 3 upper guides are 8 mm.