The tab width was made responsive based on screen size in v1.8.0. If you prefer the evenly-spaced tabs no matter what your screen size is, you can do the following:
- Launch Atom
- Open the Settings View with Cmd+, on OS X or Ctrl+, on other platforms
- Click the Gear icon next to the UI Theme box
- Go to the Tab Sizing dropdown and select Even
You can experiment with the other settings there too to see what settings you like best.
How to find the Gear Icon
How to find the Tab Sizing dropdown