Bitmap.RotateFlip()

The bitmap size wont be changed until otherwise the resolution of the image is changed. Rotate will not change the file size as resolution is not changing. But one condition, if the file is created by another application and you open the image in ms paint and changes the image then the resolution might differ. Try this.

Open MS Paint -> Create a new bit map file. Save that file -> Check the size. -> now rotate the same bit map -> Save it. -> Check the file size. Since here the same application saves the file there is no loss of resolution and the file size is not reduced.