The reason it is doing this, is to match the dropdown menu widths of a menu segment. Your 3rd level submenu width would need to match the width of the widest element (see blue outline in screenshot), and all 3rd level submenus would have to match that width. If width was not matched across all submenus on the same level, the width would jump back and forth, which would be a disturbing effect.
Now, you might ask why also the parent 2nd level has to match that width. That is a good question, but I believe it was implemented to create some "balance". Your case is a bit extreme, because you have one very long menu label on 3rd level, and 2nd level labels are all very short. If we made the width match only the same level, it would look like screenshot below. Perhaps nothing wrong with that, but in less extreme cases, it is visually pleasing to keep menu levels at same width.
Slightly related, would it be an idea to limit your menu label length? There is a LABEL setting for pages, which is recommended to use for short accurate menu labeling. By default, your menu labels are empty, so the values are inherited from your folder names (urls), which seem unnecessary long. For example:
