Yes G10 is heavy, but even if you have used a riser made entirely of G10, it just won't add up to "Heavy" in a longbow.
G10 - 110 lbs per cubic foot
phenolic - about 85 lbs per cubic foot (slight fluctuation depending on substrate)
Macassar or gabon ebony - about 72-75 lbs per cubic foot.
walnut - 38 lbs per cubic foot
osage - 54 lbs per cubic foot
Imagine a longbow riser block that is 1.5" x 1.75" x 18" = 2.7% of a cubic foot... or :
G10 - 2.97 lbs
osage - 1.49 lbs
but the above block is whole and an actual longbow riser block is about 60% waste or more depending on design.
That now makes the weight of the finished longbow riser:
G10 - 1.2 lbs
osage - .58 lbs
So, after all that math, there is only about .62 lbs or 10 oz difference between a solid G10 and a solid osage "completed" riser. That is less then a quiver and about 3 arrows.
Saying all that, it is still desirable added weight.
BigJim