Cannot Make Custom UI Issue Panel As Small as UI Kit


I’m using Forge + Custom UI to write a custom Jira app.

I notice, however, that I’m unable to make a Custom UI Issue panel screen the same (smallest) size that UI Kit allows. Here is an example of a Custom UI app and a UI Kit app that shows the size difference:

Note that even if I reverse the order that the apps are rendered, the spacing discrepancy continues:

I’ve set viewportSize: small in the manifest.yml file for the Custom UI app, so I’m already using the smallest possible size option for the Custom UI app.

How can I write a Custom UI Issue Panel app that renders as small as the UI Kit variant is able to render at?

To mirror the xlarge viewPortSize option, is it possible to add another viewportSize option to Custom UI: xsmall?

Maybe there is something I could adjust in my base index.html, or maybe its dictated by the capturing iframe?