That sounds like it, DXT requires power of two and I believe the backend upscales to the nearest power of two dimension?
We could add a warning there to warn users in advance
If they are already power of two, it goes back to a previous reply where the compression is optimising for VRAM usage, not size.
Even if the JPEG is heavily compressed, it’s not going to affect the compressed formats that much. It’s like converting a JPG to a Bitmap, no matter what the original size or compression the JPG is, the Bitmap is always going to be same size. For example, you can see the VRAM usage for both JPGs and DXTs are the same in both screenshots.