Thank you for contacting Fusion Markets. 

The reason why the product sizes, lot size, and CFDs are all different in terms of calculation because of its notional value. To help you understand it more, I will send a table of its calculation to you. However, in summary, the underlying market of which you trade often have different specifications. E.g. 1 lot of FX is $100,000 of currency and $10 per pip, but one lot of S&P500 is different and only $1 per point. 

  

I hope this answered your question if you need you to know more about how Fusion markets works, please feel free to contact us.