when i convert a tiff file into a jpeg, why does it go massive?

I'm converting a NEF file (Nikon RAW format) into a TIFF file to make it more manageable without reducing the quality of the image. When I;ve finsihed editing it, I think convert the TIFF into a JPEG for publishing.

The thing that I thought I understood, but have now tied myself up in knots with, is why the physical size of the file changes so massively, whilst the DPi reduces.

These are the respective file sizes/info

NEF/RAW Original: 300 dpi, 52cm wide
TIFF: 200 dpi, 41cm wide
JPEG: 72dpi, 174cm wide

I know the correlation is obviously between the resolution reducing but I can't seem to make it click at the moment.

HELP PLEASE - ANYONE - SOMEONE - AAAAAAAAAARRRRRRRRRGGGGGHHHHHHHHHHHH!!!