An in-depth discussion of Leaky Defenses can be found in the Carrier Battles mod tutorial pages:
DEF 200 : Damage Theory and
DEF 340 : Armor Mechanics
These pages are coming from CBmod, but the underlying game mechanics are universal. Just use your local mod's statistics and follow along.
DEF 340 in particular goes into detail of the tradeoffs between thick (large, low hp/kt) and thin (small, high hp/kt) leaky armors, plus the effects of tech level on damage absorption rates.