I have a 1920x1080 HD monitor, and I usually play Minecraft with the Standard interface scale. But on this scale, it is impossible to use IV properly, because its' interfaces look like this:

Furthermore, this is the config interface, and it is rendered with a normal size:

As you can see, the interface's size is so small that even the letters are not rendered normally, they pixelize. How do I fix this? No config options for interface scaling were found in IV configuration...

If you set your UI scale to "large" or "auto" that fixes it. Barely anyone plays on the "standard" UI option so the textures are designed for "large" and "auto" as those are much more common.

The interface with the Standard option (full-screen mode):

The interface with the Auto option (full-screen mode, it is WAY too huge):

Can you post a picture of the creative tab? Because that config GUI should be the same size as the creative tab.

There is no problem with the config tab, only with the plane UI.

I'm flagging this as a not-bug given what you've shown. In normal mode, the plane panel is the same size as all other GUI components. Yes, it is small and only takes up 1/3 of the screen, but if you look at the sizes of your normal creative GUI vs the auto-scaled one, you can see that every GUI is being scaled properly. You don't want the normal inventory to take up 1/2 the screen? Well, now that you've shrunk it to 1/2 its size, you've shrunk everything to 1/2 its size. That includes the hotbar, AND the HUD, as they're linked and part of the overlay rendering MC does.

Nothing I can do here I'm afraid. If you want tiny GUIs, then you're going to get tiny GUIs. Be them creative inventories, config screens, or vehicle HUDs.

Can you post a picture of the creative tab? Because that config GUI should be the same size as the creative tab.

I have done some investigation today. I downloaded a specific mod called Scaling GUIs. It detected only a single GUI class referred to IV, and it is minecrafttransportsimulator.mcinterface.BuilderGUI. If this GUI class scale will be set to LARGE via the Scaling GUIs, the scale that is used for Minecraft LARGE setting is NOT shown, but if the entire Minecraft GUI scale will be set to LARGE, the IV's GUI scale will become larger.
There is something strange about the scaling issue. This is that the issue appeared in a IV version later the v15.10. I had switched to 19.14 yesterday, and I had faced this problematic issue.

And one more detail: the scalability problem is present only with the following GUIs:

• Vehicles' instruments setup screen:
• Vehicles' U-Panel and the basic interface of the vehicles:
  --
  --
• All the production equipment (example):

The issue is not present in the following GUIs:

• Configuration screen (opened in the top):
• Beacon's GUI:
• Text markers editor's GUI:

There is a possibility that the problem holds inside of the GUIs' textures.

It would be better to have the problematic GUIs size in the Standard scale at least two times larger.