I would like to see a rules background that supports both lighter and heavier 'Frames, for diversity and tactical planning. A rough-cut of what I am thinking off (deffo needs thought about balancing, though):
First of all, instead of saying "4 'Frames", converting a unitsize into Drop Limits would be highly beneficial for versatility's sake. Say, we have 16 Drop Points. A Light costs 3 point, a Medium 4, a Heavy 6 points. A game with 3 'Frames would have 12 Drop Points, a game with 8 'Frames would have 32 Drop Points.
- Light
Adds 1w4/2" G, +1 bonus on Defense rolls due to size, 3 systems max, 1 structure point.
This 'Frame can carry less, but it can go much faster than the other 'Frames. - Medium
Basic 'Frame, 4 systems max, 2 structure points.
This is your usual frame, rounded out and versatile. You know the one. - Heavy
Loses 1w4/2" G, -1 penalty on Defense rolls due to size, 5 systems max, 3 structure points.
This 'Frame can carry a lot more than others, but it is also slower.

If you spend all your points on Lights, you get 4 Lights + 1 Medium out of 16 Drop Points. That's a total of 6 structure points with a total of 16 systems. They have 5 'Frames, out of which 4 add 1w4/2" G and +1 to their Defense rolls. A heavy lance gets 2 Heavies and Medium, at 8 structure points and 16 systems. While they are more durable, they do lose speed, and they have two 'Frames less. The difference in structure points is made up by the fact that the smaller 'Frames are harder to hit - and the bigger ones easier. A Lance of 4 Mediums has 8 Structure points, no bonus or malus and 16 systems, just the same. So it would be balanced, but open up the game for more tactical Lance building.
/discuss