You've encountered a well known platform difference that has been around since FileMaker first went cross platform as far as I know. I believe it's a fundamental difference between the systems rather than FileMaker. I find it's easier to develop cross platform layouts on a windows system (Got access to a parallels machine by any chance?) than on a Mac. That way you go from "more space needed" to "less space needed" and the layouts need little or no adjustment to work on both systems.
Well, it's not the answer I want but its an answer!