Equifax is currently worth about 16.6B. This takes into account the fine, so without the fine they'd presumably be worth 17.25B or so. So this was a fine of 3-4% of their market cap. Total costs associated with the breach were probably moderately higher than that.
That being said, it's important to understand that in the US, penalties are assessed based on the seriousness of the infraction, not on the ability of the perpetrator to pay.
And while it might be the opinion of lots of HN commenters that the equifax breach caused "incalculable" harms, demonstrating this level of harm in court would be tremendously difficult.