What is your Filemaker version and Operating system? If FMP12 what template? What font?
I am not sure but there might be some alignment problem. Like if any field is both top and right aligned is placed in right most place then the filed will be extend automatically to right in Browse mode. You can check position alignment in the inspector of all these fields of that layout.
Didn't you ask this question already?
Check the Autosizing "anchors" on the inspector's position tab. It sounds like the objects that are mysteriously changin size have more anchors selected than the top left pair that are the default setting for newly added objects. If you anchor an object to the right and left edges of the window, it will, for example expand and shrink in size as your window size changes.