Optimal mass works for Thrusters (only optimal engine mass = ship mass will give max speed)
Fastest speeds and acceleration from thrusters are achieved with the highest optimal mass thruster and the lowest ship weight possible.
The more weight I remove, the faster ships get, even if I am already well below optimal mass. It's been this way since Beta 2.00 and is still this way as of Final 1.03.
Probably the same for an eagle, they fall over when you shoot them, and I think they have size 3 shields.
Base figures in the specs seem to be calibrated for C class modules of the largest size the ship can mount, in my experience. With the same shield, a Viper should have almost double the shield strength of an Eagle.
I really haven't tested mass vs. shield strength in any detail.