The FIA also added a temperature limit to reduce this effect that I was mentioning. I just did some slight research on it, looks like when it was at its most effective in the 80’s, cars had a 195 litre tank, with fuel chilled to about -30c this allowed for an extra 10-12 litres, or about 6% increase.

This change was huge in that era of racing, basically meant that drivers could stay at 100% pace for longer without nursing fuel. Coincidentally, when doing some reading on this I also found that apparently chilled fuel during the refuelling era of F1 also helped increase the flow.

Anyway, that was my thinking originally lol, looks to be actually answered in the comments now though. Thanks for the reply!