I'm finally able to look at this on a computer instead of my phone. Judging by a comparison between Kyle's pics and my own, tashes do not seem to affect my visual perception of the wearer's height. Granted, we're both around 5'10" and the OP was specifically in regards to a shorter fellow...

Also, on Kyle's regulation doublet, the tashes don't come all the way to the front, but rather have peaks at the closure point that might contribute to a lengthening effect on the waist. Someone recently posted an older style regulation doublet that had tashes all the way to the closure of the jacket, as do some Sheriffmuir doublets. I wonder if the placement of the tashes has any impact on what the OP was noticing?