The size of the device has nothing to do with the proprietary formats.
Look at
the Kindle reading apps page - those are a number of programs for the sole purpose of reading the same book (same format, copy of the same file) on a variety of devices. That page has a nice photo illustrating that.
If you click through "Buy a Kindle", you can find specs, like what filetypes the Kindle supports.
You can create pdfs, but as JSWolf explained, they are really not that great for ebook readers.
And a general note: I really think you should try to do research before jumping in with questions. This is the internet, so looking up things like "which filetypes does the Kindle support" is easy. Don't expect everything being spoonfed to you. I'm sorry if this sounds harsh, but, well, you went "I never knew that!" at so many things that you would have known if you had sat down and looked for basic information on ebooks on yourself, or even just looked at the forum descriptions here on mobileread...