Caster Mastery and Contagion Question

Hello PoE people! This is my first time posting and I have a burning question. I have a friend who insists that the Caster Mastery perk "final repeat of spells has 30% increased area of effect" effects Contagion when it kills an enemy and pops. But I haven't noticed an area effect increase in practice. Am I blind or is my friend wrong?

Thank you in advance!

Is your contagion linked to a support that allows it to repeat?
If yes, then why?
If not, then no, it doesn't work, contagion in itself doesn't repeat, it spreads.

It is not, I have contagion linked to whatever supports I can find that give extra area of effect.

Thanks for the reply!