Thank you for your post.
Using a MacBook Pro running OS X 10.6.4 and FileMaker Pro 10.0v3 (and FileMaker Pro 11.0v2), I am unable to replicate the problem. When I switch from Browse Mode to Layout Mode, the width of the window stays the same, whether I have Status Toolbar and/or Formatting Bar turned on/off. This appears to behave the same on FileMaker Pro 11.0v2.
Does this occur for you with a new file? Using Object Info, what is the size of the window? What is the overall size? What screen resolution are you using? Any other information you can provide to help me replicate the issue would be appreciated.