With the current setup, everything is floating to the right and there is a huge gap to the column, which feels somewhat unnatural. In most Western countries, we read from left to right, so having everything on the right in a new tab while our brain expects the file list on the left is confusing—far more confusing than a gap to the content preview, as you mentioned.
And even if you do not have a solution for the gap yet, it would still be nice to let the user decide and add a checkbox in the settings where we could choose whether resizing when opening a new tab should be enabled or disabled, right?