".. it would make sense for the lower earner to have to take on a bit more..."
And this is why YABU.
It's not about earnings. At all. It's about time. If two people work similar hours, then the parenting load should be similarly shared.
I earn less than my DP, but I'm in a higher level role, work more hours, and have significantly more responsibility (I'm education sector, he's a software engineer) - by your argument I should pick up more of our home life and child management because he earns more than me.
He starts work at 10.30am, at home, finishes by 6.30pm, and has an hour lunch break. I leave for work between 7 and 8.30am, and finish usually by 6pm - but often 7.30pm and then drive home. And I have lunch on the fly.
He earns more, but also does more, because he also has more time.
Earnings are absolutely nothing to do with contributions at home - an hour of your time isn't worth more at home, just because it is at work 🙄.