Should Rabbet Widths be relative to the width of a Frame or are absolute values adequate?
Shifts in humidity will cause the frame to expand and contract across its width; the wider the frame the more it will expand. i.e. The wood total movement will be a small percentage the piece's overall size. So a four inch frame with a two inch rabbet will be much more likely to separate at the corners than a 2.375 inch frame with a 3/8" rabbet, despite them looking identical if the rabbet is covered by the frame contents.

Apropos larger cuts, shapers are larger than routers and spin knives rather than bits which allows them to do most of what can be done on a router table on a larger scale or in fewer passes. Large rabbets in a home shop would usually be done on the table saw or jointer though.

Using a dado stack on table saw you can take off just under an inch at a time. By moving the fence after each pass you can make a rabbet up to saw's ripping capacity. For shallower, wider rabbets you can cut them on most jointers up to the width of the cutter head. (6" at the smallest) So there are a variety of options for cutting rabbets beyond what your router does. [Technically you could hog off a huge on with multiple router passes, with sufficient time and patience.]