Why does the page size change?

Hello,

When I test my website with different tools, the parameter that indicates the page size is always different. Not very different, but quite different. I don’t quite understand how the same page can have different sizes when tested from different places: isn’t it the same content or can the ads I’m running be the cause of the change?

I also tested a page that has no ads, and I still see a difference in page size. What can cause this?

Thank you!