Thanks for reaching out! It definitely looks like there might be a bug in the software that's not allowing you to remove that extra white space from the legend. I was able to reproduce what you were seeing using a raster layer with the Unique Values renderer.
I'll be investigating this issue but if this is something you'd like to track, I encourage you to contact support. They are better equipped to handle any troubleshooting steps and gather bug data if needed. They can be contacted at https://support.esri.com/en/contact-tech-support.
In the meantime, if your legend is going to be static, a workaround would be to make the background of your legend transparent, insert a new Rectangle shape from the Graphics and Text gallery, resize it to the desired width, and place the legend on top. You can then group the two elements together (Legend and Rectangle) to keep them as one object going forward.
Hope this helps,