UltraEdit x64 version 23.00.0.59 has large icons and small icons in the ribbon. See the attached screenshot.

How does UE determine whether an icon in the ribbon is displayed as a large icon or a small icon?

How do I add a large icon to the ribbon when I customize the ribbon?

I don't think there is currently any documented way to add a large icon to the ribbon. You would need to submit an enhancement request to IDM. It seems like the kind of thing they might add in a future version, unless they are worried about people making their ribbons ugly.  8)

The format of the ribbon is stored in %APPDATA%\IDMComp\UltraEdit\uedit64.rb0, but it's in XML format, undocumented, and is obviously not intended to be edited. If you study the file, it's not too hard to figure out how to make an icon large or small. I have played around with it a little, but it's just hacking and if you didn't have a backup of the file it would be easy to mess something up. I don't think the end justifies the means.