Acceleration is a function of force, mass, resistance (drag) and inertia. The OP stated that the heavier bike accelerated harder off the line, but had a longer time to cover a fixed distance (cruising speed).

Cruising speed is basically independent of mass - on the flat anyway - but hugely effected by resistance - we can safely say that the new bike has less resistance (aerodynamic, bearing, drivetrain, etc)

We will also say that the OP is putting the same effort into both bikes.

Given the above, the only thing left for initial acceleration is inertia. ->> which leads to gearing as mentioned by [MENTION=193959]FastJake[/MENTION] and [MENTION=35181]Reynolds[/MENTION] above.