That’s not what you’d expect. I would have guessed difficulty in raising funds would be linear, but it isn’t.
The primary reason is that there are two typical investors: angels and VCs. Angels are just wealthy people who typically sums of between $10 and $100k, with $50k probably being a good average.
VCs are institutional investors who raise funds often totaling in the hundreds of millions, and are paid in such a way that they are incentivized to deploy the entire amount into investments. So, VCs like to make bigger investments because then they can make fewer. That means less due diligence, fewer board meetings, etc.
Not many VCs make it their business to invest amounts of money that small. It happens, but it’s often just something they do to lock up right of first refusal on future rounds. A lot of companies would much rather raise $500k than $5m, so they’ll do it if they like you enough, but it’s relatively uncommon.
Raising $500k from angels means convincing somewhere between 5 and 20 different people, all with their own habits and goals, to invest in you at the same valuation and terms. That happens too actually, even outside of the few syndicates that exist, but again, it’s hard to pull off.