Is this a problem with the default style settings that you have on the PC and Mac? That is, the default style on the PC sounds as though it is 12 point whereas it is set at 10 point on the Mac. Change them to match each other.
If no style has been applied to the text, then it will be using the default style settings to display it. To make sure that your document will display the same (assuming availability of fonts) learn to use and apply styles to your text... and then curse MS Word's completely **** handling of them and its appalling interface for using them. Grrr.