When you close a tab, tabs could become larger because there is now more space for each one.
But in Chrome they aren't resized until your mouse leaves that area. This way you can close multiple tabs with clicking, no movement required.
This design quirk is so useful and amazing how others haven't though of it before (once you see it it seems such an obvious idea) that some days I think about looking through the Chromium source and finding the name of the developer who created this interface feature so he/she could be properly thanked.
It was designed and implemented by
Ben Goodger, the front end lead.
When you have many tabs open in Chrome, the length of each tab shrinks so that they all fit in the bar. Smart positioning refers to, when you click a close button on one of the tabs, the rest don't expand to fill the remaining space until you move your mose away, so that the next tab's close button falls under your cursor in case you want to close it as well.