The first thing to do is look at your numbers: put a breakpoint on the line:
var newImage = new Bitmap(newWidth, newHeight);
and use the debugger to examine newHeight, newWidth, and ratio
I suspect that they will not be what you expect.
Have you considered what happens in your current image has a lower height than maxHeight?
BTW: How do you know the kb / mb size is bigger? Are you saving the new image and comparing file sizes? If so, are they the same type of file, and an uncompressed format?