By default, the size of tiles is  proportional to the video resolution.

To fix the size and expand tiles to all free space on the layout area, do as follows:

1. Go to  Settings → Options (1) → User interface (2). 
   
2. Uncheck the Adjust video frame border checkbox (4).
3. Click Apply to save the changes.
4. Reopen the Layout (see Layouts Management).

The video will retain its aspect ratio.

To expand video to all window, do as follows:

1. Uncheck the Keep aspect ratio checkbox (3).
2. Click Apply to save the changes.
3. Reopen the Layout (see Layouts Management).