On the Mac, the modifier keys to hold for resizing a node while keeping aspect ratio and center coordinate fixed are actually the same as on Windows: Ctrl+Shift.
The subtle difference here is that you can't hold the Ctrl key before you start pressing the mouse button, because on the Mac this is interpreted as the alternative combination to bring up the context-sensitive menu (popup window). Instead, you need to press the mouse button first (over one of the eight resize knobs around the node), and then press Ctrl in addition.
