It's the way Logitech designed these mice to have a lot of plastic weight in the body shell. I only ever removed a small metal plate out of my ancient MX500 device when I wanted to lighten it but it still was heavy.
Aside from breaking out a dremel you can start milling away the plastic shell until it comes down in weight, alas I suspect the shell would collapse under your hand when used .
Razer had removed a lot of weight from their DA2013 body hence it felt so cheap and nasty under my hand when used, because you could hear the squeaking and groaning when you moved the mouse about on your pad. The Chroma has indeed been beefed up with extra plastic and it's quite a solid device now. If you love the Razer Rat get the latest Chroma, you won't be disappointed (unless you really despise Synapse).