How you write the book is exactly the same in all three. It's all about what happens after the book is finished that varies.
* Control - ability to influence things like cover design, book title, pricing, marketing copy of course varies. Self you have 100% control, small press you have substantial influence, big-six you have little to no say.
* Release timing - Big six has a long lead time, at least 1 year from finished to release, but can go as long as 2 years depending if there are places in the release schedule. small press - is more along the line of 6 months. Self-publishing is just a few days/weeks.
* Pricing - ebooks priced $9.99 - $14.99 with big-six, $4.95 - $6.95 with small press, anything with self.
* Marketing - same techniques used with all three
* Editing - Big six provides both developmental and copy editing. Small press mostly just copy editing, self-publishing you are 100% on your own.
* Print books - big-six sells large volumes 10,000+, small press short run (2,000 or so), self POD and generally just a few hundred to a thousand.
* Advances - big-six pays advances, small presses usually none, self usually requires some out of pocket payments for editing and/or cover design
* Money - self publishing - earn the most if a mid-list seller, big-six - earn the most if you sell a ton of books, small press will always be the smallest earner.