reducing image size increases file size? When I download this image:


(source: ucn.edu.ni)  

then its total size is 341,0 kB.

The image has a width of 2880 pixels and height of 1100 pixel and 1 X/Y resolution pixel/in.

I opened the image with gimp and pressed scale image and reduced the width to 1920px and auto adjusted height of 733px, and keeped the resolution as it is and used the interpolation cubic. If I now export this as a jpg with 90% quality, then the image size is 546 kB! 

How is that possible and how can I avoid that?
JPEG is a lossy format and depends on both the 2d frequency components of the image and the user specified quality level.

It is possible that down-scaling an image can increase the higher frequency components and potentially result in an increase in file size, typically higher frequency components a encoded using a higher number of bits compared to lower frequency components. An example would be to imagine shrinking a checkerboard texture, the smaller it gets the more it shifts into higher frequency.  

It may also be possible that your original image was encoded with a low quality factor and your new image has a high quality factor so it can also gain extra bytes from that. On a more technical level this may be because of a mismatch in quantization bands leading to higher number of DCT coefficients being needed compared to the original JPEG. The quality parameter is not standardized and different applications have different ideas of what '90% quality' means.

Another problem is that because you are using JPEG as a source image it will introduce ringing artifacts into the image, which then need more bits to encode when trying to encode it.

Unfortunately there is no easy fix for this apart from reducing quality.