The ratio holds; everything around it moves
Doubling leaf and water keeps the ratio constant, which is the part people check, and changes at least four other things at once, which is the part they do not. A larger vessel cools differently, presents a different geometry to the leaf, takes longer to fill and longer to empty, and holds the leaf in a deeper bed. Each of those has a modest effect and they compound. The usual outcome of scaling a good small brew straight up is a large pot of tea that is simultaneously flatter and grippier than the original, and it gets blamed on the leaf.
Bigger vessels lose heat more slowly
A large volume of water has a smaller surface relative to its bulk than a small volume, so it sheds heat more slowly. That is helpful for a black tea in a big pot, which stays hot enough to keep extracting properly all the way through. It is unhelpful for anything delicate, because a small mug of green tea drops through its danger zone quickly while a litre of it does not, so the same nominal steep runs at a higher average temperature in the larger vessel. If you scale a green tea up and it turns bitter, this is the most likely cause.
The leaf bed gets deeper
In a wide shallow vessel the leaf sits in a thin layer with water reaching all of it. In a tall narrow one the same doubled quantity forms a deep bed, and water circulates around it more readily than through it. The leaf at the centre extracts late and incompletely while the leaf at the edges over-extracts, and the resulting liquor carries both faults at once. This is the same problem as a cramped infuser, arriving by a different route, and it is why a wide pot scales better than a tall one.
Filling and emptying become part of the steep
Pouring a litre into a pot takes appreciably longer than pouring two hundred millilitres, and emptying it takes longer still — so the leaf at the bottom of a large pot has been in contact with water for meaningfully longer than the nominal steep by the time the last of it is out. At three minutes this is a small percentage. In short-steep brewing it is fatal, which is precisely why gongfu vessels are small: the pour has to be a small fraction of the contact time, and that constraint sets the vessel size rather than the other way round.
What to do instead
Brew twice rather than bigger, which is the answer nobody wants and the one that works. Failing that, scale the vessel’s width rather than its depth, shorten the steep slightly to account for filling and pouring, and use a slightly lower temperature for anything delicate to offset the slower cooling. Decant the finished tea into a second warmed vessel rather than leaving a large pot standing on its leaf, since the larger the pot the longer the tail of over-extraction while it is being drunk.
The scaling that does work
One family of methods scales cleanly, and it is the one built for it: brew a concentrate and dilute at the point of service. Samovar and çaydanlık service, and American sweet tea, all separate the extraction from the volume, so the quantity being brewed stays small and controlled while the quantity being served is whatever the room needs. If you regularly make tea for a crowd and are unhappy with the results, the fix is probably to change method rather than to keep enlarging the pot.