- They are larger and slower than light 'mechs.
- They don't have the armour or firepower of heavier 'mechs, but are the same size.
- Laser streaming (or Lights are just too damned small).
Because of the streaming effect of lasers, combined with the high speed and small size of the light 'mechs, it's extremely difficult to use lasers to kill light 'mechs; you need to use LRMs, S-SRMs, larger ACs and PPCs in order to do this effectively.
You can mount all these weapons on bigger 'mechs and in reality, it's easy enough to get the range you need to be able to target light 'mechs - why bother using mediums when you can use a heavy/assault and a bit of skill?
The size difference between lights & mediums is important, because it affects the relative firepower. That is, firepower is a combination of damage and accuracy; a light is relatively more accurate against a medium than the other way round, because the light is smaller (assuming equal skill of pilot).
Sorry, but size DOES matter.
Suggestions:
- Make light 'mechs larger i.e. the same size as medium 'mechs, only slightly skinnier in the arms and legs.
- Reduce the streaming time on medium lasers from 1 second to 0.25 seconds or less. This would not only make medium lasers more effective against the small 'mechs, it would also improve the ability of light 'mechs to hit & run.
- Give light 'mechs a role OTHER than ninjaing assault 'mechs in the back; make them able to so something that means they don't need to expose themselves to fire from big 'mechs so that they actively have to be hunted.
- Restrict engine sizes to the 'tonnage multiple'. More radical, but it would mean that you're putting a more drastic cost on speed improvements - a 70 ton 'phract could only increase speed by going from a 280 to a 350 (can't put in bigger, because of the 400 maximum).