Its just a guess but perhaps the increase in font size is increasing the overall dimensions of the Map which means for it to fit the same print boundaries it gets sized back down.
You could try keeping the font size at 12pt and closing down spacing elements such as Topic margins and Topic spacing to effectively make the Map smaller which may then mean it will get an increase in size when it grows to fill the print boundaries.
Just a guess at the moment though but worth a try.
The MindManager Solutions Specialist
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More thoughts: Margins and padding are in mm. Changing font sizes changes the relative size of these settings to the font. Similarly other distances which you cannot set are "fixed" e.g. the invisible padding around each topic.
A method I have used to control print size is to place Floating Topic at diagonally opposite corners. As you move these away from the map centre the printed size of the map will reduce.
You can also use a background image with floating topics to help you size the map: Use Background Images to size your maps and Using Background Images with MindManager