If your sprite is a single horizontal line, resizing it to smaller than 100% size is ignored, it is resized to 100% instead. Same is true for single vertical line, apparently what matters is that the area of the axis parallel bounding box is zero.

Reproduction: https://scratch.mit.edu/projects/534726354

My browser / operating system: MacOS Macintosh X 10.15.2, Chrome 89.0.4389.82, No Flash version detected

I think this is because the lower size limit is active (the line is 2 pixels high according to the costumes tab).