Watterson convered this in some of Calvin and Hobbes reprint books, but basically, a Sunday strip is at most 12 panels (call it 12 units) long. By modern convention, you have a 4-unit title panel, a 2 unit panel with a single throw away joke (or two 1 unit panels with a throw away joke), leaving 6 units left for the main Sunday joke.

A newspaper then has several options for displaying the Sunday comic. They can run the full 12 units. They can eliminate the 4-unit title and run 8 units. They can also substitute the 4-unit title for a 1-unit title and run a 9-unit strip. They can drop the 2 unit throw-away joke for a 10 unit strip. They can drop the title and throw away joke for a 6 unit strip (smallest they can run).

After Watterson's sabbatical in the early 90s (I think it was 1990), he fought for a full 12-unit (in a 4x3 layout), and won (since his strip was so popular), in which he was able to work without any constraint. He could basically do a huge 1-unit Sunday strip, or a 24 small panel strip, or strips without panels, or what ever he felt was needed for the Sunday strip. The artwork for the late Calvin and Hobbes Sunday strips was phenomenal and probably nothing like it had been seen for 60 or 70 years (and nothing like it since).

Yes and no; IIRC the XKCD chap says he makes his money from merchandising. But you're right; people who want their content syndicated set up RSS on their sites and people who don't, don't.