I think this is how it works!
Here is a Steem Power Calculator I came across!
The more steem power you own the higher the percentage rate of interest.
For example if you have 10 steem power you will get an additional
2.185 steem power in 30 days
If you have 100 steem power you will get 21.85 in 30 days
If you have 1000 steem power you will get 218.5 in 30 days
and so on
Here's the calc https://jscalc.io/calc/KcEkRo6qv2s8CM6S

https://steemit.com/steem/@deeprocks/steem-power-interest-calculator-updated