Operating system: Darwin-18.0.0-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro 580 OpenGL Engine ATI Technologies Inc. 4.1 ATI-2.0.36
Broken: version: 2.80 (sub 57), branch: blender2.7, commit date: 2019-04-16 20:13, hash: rB5d8030b8f3cf
Short description of error
When using a grease pencil material based on a .png image to draw strokes,those strokes look far too thick in the 3d viewport when the grease pencil object is scaled down or when the camera is moved far away from the drawing. When zooming in or enlarging the grease pencil object beyond some threshold, the strokes appear to be the correct thickness in the viewport.
See attached an image of a grease pencil object in the viewport next to a render of the same file (scaled down to match the viewport image size). The lines in the render are the expected thickness, the ones in the viewport are too thick. And below that an image showing the material settings used to draw the image.
Exact steps for others to reproduce the error
See the attached blend file I used to make the test above.