The KB Protection form the Special Global IOs (Steadfast, Zephyr, and Karma) are named after the IO, just like Luck of the Gambler's 7.5% recharge bonus.
The KB Protection that comes from IO Sets have the generic small/large/etc... naming convention. 3-slotting Kinetic Crash and having the Steadfast Special looks like this under Real Numbers Combat Attributes:
Steadfast Protection: Knockback Protection +4.0 from Self
Small Knockback Protection Bonus +3 from Self
The other sets that give a KB Prot are from the PvP sets and they're also +3 protection. So, all we know is that +3 KB Mag Protection is called 'Small.' *If* KB Protection is not on a unique scaling system, then the base KB Mag Protection is 2.
Base 2 x 1.5 Small = +3 Mag Protection.