Not quite sure how to explain it, but let me try again.
First of all, the time needed for 1 Growth cycle increases by +X seconds for each fairy level above lv27. I believe it's 5 seconds, so let's use that as an example.
Okay, fairy's now lv28. It now takes 65s for Fairy to obtain 1 Growth, but here's the catch. It takes the same amount of time for it to use up 1 Stamina, 65s. Just like the time needed for 1 Growth increases, the time for 1 Stamina to get consumed increases as well.
In other words, a Fairy with 6480 Growth will still need 6480 Stamina, except, as the levels go up, the time needed for that Stamina to get consumed increases.
Let me know if you need any further explanation with this.