I'm trying to merge 60 geotiff files into one huge file in order to be able to product tiles from that resulting huge geotiff file by using gdal2tiles.

The merge process worked, but for some reason it produces a file that has a size of 78GB instead of the expected 19GB. Not quite sure what went wrong there.

The reason for trying that is because tiling the 60 geotiffs one by one gives me white spaces on the map border.