Only if the letters have thickness. If they are just 2 dimensional lines (which is the minimal information to construct a letter), you'll have to shrink it to infinity into a single point.
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.
Basically any convex shape has a big/small size configuration in which one doesn't fit in another
Or in other words: if you can't draw a line between the center and the edge that intersects with another edge, the shape is guaranteed to fit a smaller version of itself