The story says 2,000 agents were involved. Do you trust that number? I'll assume so through the following reasoning: I'm not sure how many agents they have, but
https://corporate.westernunion.com/ says there are 500,000 Agent locations, so let's conservatively say there are 500,000 agents (surely there's at least one per location). That suggests (at most) 0.4% of their agents were involved. I doubt 100% of those agents' work involved aiding and abetting scammers, so less than 0.1% sounds totally believable to me.
That said, I think arguing about how big the fine should be is accepting the faulty premise that a fine is an acceptable penalty. Wire fraud is a felony. I know someone who went to prison for it. The article says "[Western Union] agents were helping scammers avoid detection" and "Fraudsters [...were...] giving Western Union agents a cut in return for processing the payments". They willingly participated in felonies for personal profit. Shouldn't they—and all the supervisors who can be proven to have condoned their actions—be going to prison? Or is this okay just because everyone's doing it and they have a fancy corporation to hide behind?