I just found out that the cell size of raster change after I rotate or georeference it with Georeference. Is there any explanation why it works that way?
If you have a regular grid of points aligned with your X and Y axis, and you rotate it, then the distance between your points parallel to the X (or Y) axis will change. This is the same with pixels, because they are resampled as rectangles with their sides parallel to the axis.
As for the georeferencing, it can include a change in scale, so obviously the pixel size might change. This change in scale depends on the projection (some projection will locally preserve the distances, but most don't)