The Shuttle SRB design still was dictated at least in part by the segments being transported by rail - each individual segment could be only as heavy, wide and long as the cobined stretch of railway from the factory would allow.
It influences other rockets as well - for example the Proton:
https://en.m.wikipedia.org/wiki/Proton_(rocket_family)
The long thin tanks you see mounted on the first stage are not drop tanks or additional boosters, its the tanks holding the first stage fuel, with the central core holding all the oxidzer.
Thats because the core stage is limitted in width by what you can ship to Bayokonur by rail, especialy IIRC one specific rail tunnel on the route. So they ship the core stage and the additional fuel tanks on separate rail cars and then bolt them in place on the cosmodrome.
In similar manner Falcon 9 is limitted to 3.9 m of width as that is the maximum you can ship over the US highway network without special care.
If you want to go bigger, you need special aircraft and barges, or build the thing in place like it is currently being done with the SpaceX Starhip.
