Re: "According to my math, my IMessages documents on my iPad are only ~6.2 gigabytes big, while the data accompanying it is a whopping 15.55 gigs on top of that."
Wondering if you have taken into consideration all the attachments like photos, audio, etc. that people send with messages. (.Eg: Photos may be viewable in the "Shared with You" Album in Photos. Media can take up space. It depends upon your settings.)
Use Shared with You on your iPhone and iPad - Apple Support
Eg: says:
"Content that's shared with you in the Messages app is organised automatically in a Shared with You section in the Music, Apple TV, News, Photos, Podcasts and Safari apps."
and... if needed see how to:
"Remove shared content
- Go to the Shared with You section of the app.
- Touch and hold the thumbnail for the content, such as a News article or a Podcast.
- Tap Remove.
Depending on the app, you may see different options for removing the shared content.
To customise the shared content that you see in Shared with You in the corresponding apps:
- Go to Settings > Apps > Messages.
- Tap Shared with You.
- Choose the apps where you want to see shared content."
