Just happened: I saved a PNG file of size 155px * 198px and with transparent parts and the result is an image with way too high file size (1.3MB, see [...] ).
When replacing that transparency with any color its size is as planned (12KB, see [...] ).
I'm using Paint.NET 4.0.4, just checked Paint.NET 4.0.3 and got the same problem.
Am not sure, if this is a real bug or my fault, just tell me.