2 iPads, same Apple ID and one has double the storage then the other?

Hi yonbucher,

Thanks for posting in Apple Support Communities.

For the music storage on the older iPad, go to Settings > General > iPad Storage. Click on Music. Check at the top to see if Documents and Data take up a significant amount of music storage. Connect iPad to your computer and open Finder. Click on your iPad icon from the left-hand sidebar and it will show your iPad storage. Check to see if this storage matches what shows on the older iPad.

You should see a screen that looks similar to what shows in the section "Use the Finder or iTunes to check the storage on your iOS device" of How to check the storage on your iPhone, iPad, and iPod touch.

"Use the Finder or iTunes to check the storage on your iOS device

1. On a Mac with macOS Catalina 10.15 or later, open the Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes.
2. Connect your device to your computer.
3. Select your device in the Finder or in iTunes. You'll see a bar that shows how much storage your content uses, divided by content type. 
4. Move your mouse over the bar to see how much storage each content type is using."

Regards.

Hi yonbucher,

To clarify a few points and see if there might be any other possibilities, can you let us know more about the music on your devices? It sounds like they were either iTunes Store purchases or downloads from the Apple Music service. Is that right?

Also, when you checked the amount of songs on each device, was that under Settings > General > About, or Settings > General > iPhone Storage > Music? Do you get the same result when you check both of those locations?

While updating is generally a good step in resolving storage issues, if you're unable to update for software compatibility reasons, checking to see if the storage matches in all locations on the same device will help in working out if any other steps might be available.

Cheers.