Assuming we shrink all spacial dimensions equally: With Z, the diagonal will also shrink so that the two horizontal lines would be closer together and then you could not fit them into the original horizontal lines anymore. Only once you shrink the Z far enough that it would fit within the line-width could you fit it into itself again. X I and L all work at any arbitrary amount of shrinking though.
Assuming we shrink all spacial dimensions equally: With Z, the diagonal will also shrink so that the two horizontal lines would be closer together and then you could not fit them into the original horizontal lines anymore. Only once you shrink the Z far enough that it would fit within the line-width could you fit it into itself again. X I and L all work at any arbitrary amount of shrinking though.
T, V, Y can be shrunk by any amount and still fit aswell! Possibly even K depending on the font.