Fwiw..
Re: economics/quality/tir..
What happens is that a bigger lathe, say a 12x24 vs a 10x, will be much more rigid, and much more basically accurate, with the same basic manufacturing-quality level, IF it maintains the same mass/length, +/-.
This is why no small good lathes exist.
It is very, very expensive (not really hard, just extremely tight tolerances on very short contact areas/wear pads, etc) to make a superb quality 9x lathe, or a 200 kg mass 9x, like The Schaublin 9x, 54.000 € new , now, 3rd company after 2 bankruptcies.
It is quite cheap and easy to make a more rigid, more inherently precise, lathe of say 12x24 size, or more, at 450 kg mass, or more.
When You go from a 9x to a 12x, especially a more industrial 12x, the ways are (very much) wider.
The bed casting is (much) taller in the vertical direction, leading to 2-3-5x more rigidity off-hand.
The saddle h-shape is wider, and deeper, by about 30-50% (and has more surface area).
Cube it for rigidity 1.5 (wider, deeper) pwr 3 == > 3.375 times more rigidity and vibration dampening x surface area, PER AXIS, == 5-6x better, for the same build quality.
Example:
If a 9x lathe has 0.01 mm slop, on the z axis guideway, at say 200 mm ends.
12x, 260 mm. 0.01 mm, 20% wider (mine == 300 mm, => 50% wider).
1.2 pwr 3 =1.728 more rigid, just on ends.
== 20% wider,
=> 2.07 more rigid, on z-endpoints analysis only, ..
Add depth in x or width of ways, similar 205 (or more), another 2.07, => 4.3 x more rigid overall.
Also, on oil-lubricated surfaces, the surface area has a huge vibration = finish quality, effect.
The oil film has dampening effects that go up pwr 4 by pressure and area.
So a 12x vs a 9x has inherently about 4-10 more dampening from the oil-film effect, with typical lathes as made today.
Of note..
All the famous best manual lathes,Monarch10EE, HLVG, Sharp (copy) etc..
have had much longer and heavier headstocks, heavier construction, wider ways in x, wider saddle in z, by about 30-50%, as compared to typical ones.
This leads to about 20x more dampening, and maybe 10-30x more rigidity (hard to quantify).