It comes down to fluid flow.


Drum brakes take more fluid to operate, the piston in the wheel cylinder has to move further because the shoes are retracted from the drum after each use.
Drum brakes of any normal size use 1/4" line or larger yet on big trucks. It'll be a small drum brake that uses 3/16", cars and light trucks.

Disc brakes don't have return springs, the pads are always skimming the rotors and very little fluid volume is necessary to apply the brake. That's why many of them use 3/16" line. Still there are exceptions, I've used huge dual piston calipers on medium duty trucks that have (came with) 1/4" brake lines.

Then there are situations like a rear axle with disc brakes could be fed with a single 1/4" line and branch out into a different 3/16" line for each side.

Some folks tell you as a rule of thumb that 3/16" is for disc and 1/4" is for drums... That's misleading at best.
Much more accurate as a rule of thumb to use what the original vehicle used (the vehicle that the brakes you're using came from).

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-