Question is what you want to do.
Combat Ships require Big Shields, Power Links etc.
Since you.ll be using your Weapons and Shields.
As well as your Armor.
Sensors and other stuff however are useless weight and its recommended to buy the Light Variants (usually D Class)
An Explorer for example will mostly need Sensors and Modules for this.
To retain high Jump range its better to keep anything on light modules with the exception of Discovery Scanners and Frameshift Drive
Cargoships are fine with pretty much everything on lightweight except for Frameshift in order to get as good Jumpranges as possible when fully packed
Generally you can see the Values upon Upgrading your Ship.
The Values resulting are shown down right Corner.
If you worsen your Values by Upgrading your Ship then simply dont do it.
For example.
I switched to C Class Thrusters from B Class for my Viper.
Because the Lighter Thrusters actually gave me an Boost which was worth more than the better Thrusters.
The D Class Thrusters are lighter but too weak for my pretty heavy equipped Ship.
Greetz