Nano Banana 2 only supports 1K resolution (1024x1024) natively. Anything above that is upscaling. So this matches SDXL. GPT Image 2 does support 4k natively (but experimentally).
It's not upscaling for NB2, 4K outputs are very different from 1K, and output tokens count is also different.
Where did you get that info from? According to Google's own docs as well as my own image generation tests via the API, it supports up to 4K natively for gemini-3.1-flash-image-preview (aka NB2).
It just defaults to 1K. But I didn't see anything in the docs stating that it's just a simple upscale for larger resolutions.
https://ai.google.dev/gemini-api/docs/image-generation#gener...