Okay so, I've come to realize that a character's counter move has a damage threshold. This threshold signifies when a character's counter no longer deals the base damage & then applies that countered move's damage & the counter's multiplier. So. . .
DISCLAIMER: Not 100% correct, but pretty close.
Both Lucina & Marth's Counter deal 8% base damage normally, but if the move deals ~7.5% damage or higher, then their 1.2x multiplier applies.
Ike's Counter deals 10% base damage normally, but if the move deals ~9.166666664% or higher, then the 1.2x multiplier applies.
Shulk's Vision counter deals 10% base damage with the regular attack or 13% base damage with the Forwarded counter, but if the move deals ~6.166666665% or higher, then the 1.3x multiplier applies & both the regular & Forwarded version deal the same damage after passing this threshold.
Both Little Mac's Slip Counter & Palutena's Counter deal 10% base damage normally, but if the move deals ~8.466666664% or higher, then their 1.3x multiplier applies.
EDIT: I do not know how I would figure out the threshold for Mii Swordfighter's Blade Counter due to the weight-dependent factor as of now before testing it. So eh I guess.