When i rectify a geotiff which is already 24 bit color, the new geotiff is 3-4 times the size.

murrayclift

I only have the option of outputting 24 bit (which is what my geotiff already is!) Why does the size increase by at least 3 times no matter what I do...? these geotiffs I am rectifying already have co-ords and all I am doing is adjusting.. I have no idea why they are so huge!  this is on version 15 it looks like the same on 19 from what I can tell