Please check the gif attached first so it's explained better.
If you use Google Chrome and open a lot of tabs, you will realize that when you hit the x button next to the tab you want to close, the other tabs move to the left so your cursor is always over the x button of the next tab in case you are closing those other tabs. When you move the cursor, those tabs are then resized.
In AC19, tabs are resized when there are several of them opened, but don't have that nice behavior when closed. With this, it would be easier to close a lot of tabs for a while to use less memory.