Make your button height to allow 2 lines, and button width to accomdate 2 line formating with maximum desired name.
Make multiple buttons for different widths and make the bad ones invisible when not used.
I only see this spontaneous "shrink down" on buttons from a file converted to .FMP12 from .FP7 and then only if the button was created prior to the conversion. Newly created buttons don't seem to show this behavior.
This has previously been reported as a bug in Report an Issue, but surprisingly, FileMaker Inc techs do not classify it as a bug.
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thanks PhilModJunk, your comment helpes indeed.