I use Apple Keynote for all my presentations (and I create a lot of presentation decks). It is by far the best presentation platform out there. It beats the heck out of Microsoft PowerPoint in terms of ease-of-use, features and precision control. My decks have hundreds of slides and tons of graphics. And I always wondered why my Keynote files were so large. Many of my decks are 500 MB or more.
Recently I found the culprit: When you copy and paste an image/photo from a photo editor (in my case Pixelmator) into Keynote, it gets pasted as a TIFF. Presumably Apple wants to preserve any possible transparency in the image as well as the image quality. But it also leads to very (VERY) large files.
So here is the pro-tip for everyone using Apple Keynote and editing their images in an external editor: Never copy and paste them into your Keynote presentation but rather save them as a JPG or PNG (if you need transparency) and then drag & drop the resulting file into your deck.
I went through one of my larger decks and pulled out all the TIFFs which Keynote inserted upon copy & paste, resaved them as JPEGs and reduced the file size of the deck from nearly 2 GB to about 700 MB (it is a deck with about 800 slides).