I'd have thought the same, so I'm also surprised it's just custom not an actual rule -- though never underestimate the power of custom and the social disapprobation which would follow from flouting it! -- and I suspect the French originally did have their own version of the medieval bread assizes, whereby everything was more strictly regulated as to weight and/or price.Relating to TJ’s discussion, I had always understood from my wife, who was French, that the rule for the weight and size of baguettes here in France was very strictly controlled.
Almost certainly, though I can't pin down exactly what and when -- according to the Federation of Bakers:Is the format of a TIN in British bakeries a standard? Or was it just so much part of my youth that I projected a fixed size local loaf all round the world.
Until 2008 legislation required all bread sold in the UK to weigh 400g or multiples thereof.
so undoubtedly there would have been a similar rule in the 50-70s, perhaps expressed in the size of tin, but more likely as to weight. And though bakers are now allowed to sell in different weights, there are still a plethora of rules to which they have to abide, though perhaps not as complicated as this medieval gem:Assisa Panis (Assize of Bread): When a Quarter of Wheat is sold for 12d., then Wastel Bread of a farthing shall weigh £6 and 16s. But Bread Cocket of a farthing of the same grain and bultel, shall weigh more than Wastel by 2s. And Cocket Bread made of grain of lower price, shall weigh more than Wastel by 5s. Bread made into a Simnel shall weigh 2s. less than Wastel. Bread made of the whole Wheat shall weigh a Cocket and a half, so that a Cocket shall weigh more than a Wastel by 5s. Bread of Treet shall weigh 2 wastels. And bread of common wheat shall weigh two great cockets.
which reads like a fiendish logic puzzle! (This is from https://www.engr.psu.edu/mtah/articles/pdf/bread_assizes.pdf which gives a helpful summary of what the different types of bread were.)