You can compare total better than per user at these scales.
Lemmy needs a certain amount of performance to keep up with federation, but once you have all the images and posts and comments you don’t need second versions until you scale to a size that mandates multiple machines. Which I would guess is more in the 6+ digit user range, where you start averaging requests per second not minute.

In some sense, every lemmy user is a user of your instance via federation. You need to pay the performance for all 100k of us whether your instance has 10 or 10k of those. Local users are just a bit extra demanding on your hosting resources.

I suspect the bias we see here with larger instances paying a bit more (50-ish instead of 10-ish) is more due to reliability and snappyness than actual performance needs too. As in you don’t have to you just can be bigger, so you tend to get some of the smaller pricier perks you might not go for for a smaller instance.