Why does rotating a TIFF in Preview increase the file size?
So when you make any changes to a file and 'save' it in Preview, the file is re-encoded.  It must be using different settings that the program that originally created it.

It has nothing to do with rotating.  If it did you could try rotating it back and the file size would drop, but that's not the case.

I just tried saving a TIF and I can see that Preview seems to be adding an Alpha channel to the image.  Not sure if that would be the reason for the increase, but it is something that is different.