What accounts for the size of a backup?

What accounts for the size of a WP Clone backup, and is there any way to reduce it? Here’s the scenario:

I have 3 very similar sites to develop – same design for all, but changes in content. I did most of the initial setup on a development domain so that I could have a template, then used WP Clone to copy the development instance to the first of three real sites. This backup was reasonably sized.

The client came back with a few changes and since I’ll have to mirror those out to the other 2 sites, I went back and made the same changes to the development environment and went to clone it again. The development clone is now 1.2 GB – it’s huge! I had to increase my PHP memory limit and timeout just to get it to finish.

The REALLY weird thing is that I made a clone of the now-completed first site, where I’ve added a bunch of content, just to compare. That one’s only 500 MB. How can two sites with identical plugins and themes be of such radically different size, especially where the one that has many more posts and pages and a lot more media is the smaller of the two, by almost half? Is there anything I can do to cut down the size of the template clone? I’m concerned it might be too big to be restored without timing out the server.

Thanks in advance!

https://wordpress.org/plugins/wp-clone-by-wp-academy/