What do you mean it gets thinner? It should move from right to left. For vertical it's not capable of that for now but I'll implement that in the future version.
Viewing post in [ANSWERED] What about vertical bars?
Moving right to left, getting thinner = kinda the same thing in my case, lol, because the bar I made was 100 pixels tall but only about 15 pixels wide; I was going for something more like the energy bars used in the old MegaMan games. Before trying out your plugin, I was using the Variable Gauge plugin from Visustella, which has options for both horizontal and vertical placement... But it seemed to conflict with some other plugins that I needed, so I started looking for alternatives. Yours works, but apparently just for horizontal bars.