The shields just modify the base shield strength defined by the ship itself. A Python has a base strength nearly 3 times as high as an Asp's. The difference in optimised mass and actual hull mass will alter the modifier that is applied to this base strength. If the optimised mass and hull mass are the same you get the base ship's shield strength. Higher rated modules will allow the modifier to shift more favourably in both cases of hull mass being lower or higher than optimised mass.
Thank you very much for the insight into how shield strength is determined. I did a lot of testing a bit ago trying to figure out the specifics of shields, but was unable to come up with a formula to reliably determine shield capacity for any given ship/generator combo. Do you think you could provide the formula used? The fact that I couldn't quite figure it out bugs me. I (and I'm sure many others) would be eternally grateful.
My shield research thread, if you're interested in seeing what we (at least think we) were able to piece together: https://forums.frontier.co.uk/showthread.php?t=89770
Any chance of getting that formula, or any other specific bits of data? Pretty please, with a cheery on top.