Also, there's the expectation that beta software WILL contain bugs, incomplete features, and unexpected results... because the point of the beta process is for Apple to get responses and reports of the state of the software at various points along the development process.
Per the release notes:
Software Update
Known Issues
- Your device might restart but isn’t updated to iOS & iPadOS 14 beta. (64606517)
Workaround: Try removing content to free up additional storage or update your device using your computer.
I'm wondering if there's some kind of extraction/compression issue going on. It's sure to be resolved given how Apple likes to minimize installer size.
Also, this expectation of beta software being bug-free is getting out of hand. This isn't targeted at anyone in particular, or maybe it is, towards those who hold this expectation. I've had betas that have refused to complete installation, got stuck in DFU mode and have had to wait two weeks for the next seed, with an unusable iPad. A storage bug is far from the worst.
This is the risk you take with running development software and it's completely optional to use. I get it, its new stuff and cool to have before everyone else but please stop treating it like it's production grade.
If you have bugs, thoroughly report them and wait patiently for the fix.
If you can't handle that, don't use it.