If you use an external mouse that has a scroll wheel with your Mac, you can create a file named Default (OSX).sublime-mousemap in your User package with the following contents in order to be able to do this:
[
{ "button": "scroll_down", "modifiers": ["super"], "command": "decrease_font_size" },
{ "button": "scroll_up", "modifiers": ["super"], "command": "increase_font_size" }
]
This doesn't work with the trackpad or with a magic mouse (which seems to just simulate a track pad) because scroll touches on the trackpad don't generate scroll wheel messages.
