Heavier bikes are better for higher speeds and are more stable when a car passes you at 45mph.
I know the Vespas had some pretty light bikes except the Grande.
When you rev your engine up, is there no change in speed while your driving at 20mph?
It may be an improper jet or it may be your clutch is slipping when you give it the gas.
You have to give us more info and you have to do more tests.
Best of luck,