I've heard the B5 is pretty light too... maybe your Assomiya front end is lighter?
All kidding aside, have you weighed both cars without tires too? Tires can make a lot of difference, I recall a fellow club member weighed his Astro tires (Schumi Cut Stagger front w/o insert, Schumi MiniSpike 2 rear) at 180g, whereas his clay tires (ProLine 4-rib front, Proline Caliber rear) at 235g. And then you're not even taking into account the debry and sand that might accumulate in the inserts after some use.
Oh, and before I forget... Neat build and electronics setup!
