Well... I was surprised to see that while the Spectral LAM conversion equipment takes up mass(10 % of the Spectral LAM's mass) it isn't shown on the Spectral LAM's recordsheet.
So my personal theory why the Spectral LAM's cannot use lighter,but, and this is important, also bulkier, equipment(Endo Steel Chassis, Ferro-Fibrous Armor, XL Fusion Engine, XL Gyroscope) is as follows:
The LAM conversion equipment also takes up space on the Spectral LAM(similar to the Standard Fusion Engine) thus limiting the space available for other components and prohibiting the use of the lighter but also bulkier equipment like Endo Steel Chassis, Ferro-Fibrous Armor, XL Fusion Engine, XL Gyroscope etc.
Besides that I agree with Dread Moores in that LAM's using all that lighter equipment would indeed be pretty ridiculous.