Remember the estimate is only a guess: the only way to know for real is to actually export your project, which will run various CPU intensive compression algorithms and compact the project size down. More info in this blog post. The estimate is pretty crude since it doesn't do all that work.
When you import images, regardless of the format of the image, C2 stores it in the project as PNG-32. This is what the estimated size measures, so there should be no difference between importing a PNG-32, a PNG-8 or a JPEG on the estimated size.
You can set a different export format in the image editor. It's still PNG-32, but will be recompresed to either PNG-8 or JPEG upon exporting if you use this option. It's not automatically set, so if you import a JPEG it will still export a PNG-32 unless you click the image format button and change it to JPEG.
The export format is not currently taken in to account in the estimated size, but for the next build I've changed it: PNG-8 will be estimated to be half the size, and JPEG will be estimated to be a quarter the size. Again those are pretty arbitrary guesses, so you'd have to export to know for sure.
Hope that helps!