Is it possible that the TIFFs all have a (small, high compression) preview image embedded? Maybe that ends up as *.png.1. IIRC one TIFF-file can hold more than one image.
Update:
yay, Corion was faster....
Comment on Re: Image::Magick writing more files than I want.
You are both correct. The TIFFs contain a preview image that I completely forgot about! Image::Magick was actually scaling those tiny images UP, which explained the awful resolution. Thanks for the help.