That makes sense - thanks!

https://wiki.factorio.com/Oil_processing#Advanced_oil_processing

Under Advanced Oil Processing, it says the optimal ratio is 5:1:7. That means:

5 Refineries turn crude into 5 Heavy Oil, 22.5 Light Oil every 5 seconds
1 plant on Heavy Oil Cracking turns 4 Heavy Oil into 3 Light Oil every 5 seconds ...