How to tween lines

Hi,
I know this is a pretty stupid question but…
THis is what i wanna do: Create a small line;

--------- ,

and make it so that after 15 frames it is longer;

------------------------, (lol)

but when i do this, the supposedly longer line is also alot more thicker,

Q: how can i make the line longer w/out makign the width longer???

Thanks for any respones in advance :slight_smile: