Why do I get a smaller file size when I increase the pixel count in GigaPixel

When I increase the size od my photos with Gigapixel why does the file sixze plummet?

I have just increase a photo of a bird which was 5000 x 3334 [16.6 mP and 6.42 mB] to 6500 x 4334. The Pixel count has increased to 28.17 but the file size has dropped to 2.68 mB

I have it set on High Quality. Is this to be expected and is there any way I can increase the file size [which, I assume, means more data stored]

Gigepixel not only increases the resolution but also does a cleanup / noise reduction. The latter leads to smaller file size (but often better image) as that noise hinders jpg compression. So even though the resolution goes up the file size can be lower due to better working compression.

So, if you don’t see compression artifacts in your saved image, everything’s OK.