After you’ve finished your design in Illustrator, you might want to scale down your design to a smaller size. But in doing you might realize that Illustrator is scaling but not scaling the line widths of your design in proportion; so you get something like the gif below.
To fix this and have the line widths scale in proportion to the whole design and retain detail, go to Illustrator > Preferences > General…. Then select the General option in the left column menu and make sure to check Scale Strokes & Effects.
Now when you scale your design, you should see your lines retaining it’s stroke width in proportion.