How come the file size of a Publisher document fluctuates so much?

[Intuos5]

Something I have been wondering for a while is why my file size changes so much between consecutive versions of the same document (book)? I have been continuously adding images, text & pages and notice that my file dropped from 25 MB to 18 MB just now, despite filling a text column and adding image. I don't have save history enabled, so it's kinda weird to me. Whenever the file gets smaller, it also performs quite a bit better. The difference is definitely noticeable — the document goes from lagging when typing to fluent.

[MikeTO]

This is the way most apps work but you don't notice it with other apps because you're not adding so much content to them at once as you do when you place a large image in Affinity.

Apps don't save the entire file from scratch each time you save, they do an incremental save, saving only the changes from the last save. This saves time so you can get back to work immediately but it makes the file more complex and overtime it gets a bit fat. When it reaches a certain point the app does a full save which reduces the file size. You can do a full save at any time by choosing File > Save As instead of File > Save.

Cheers