Just some kind of multiplier to the base.
Ikkaku has a growth of 3 and let's say the base is 35, upon a level up he gain 36.05 point of strength (35 * 1.03) then 37.1 at the next level (35 * 1.06) and it keeps going until level 159...
don't quote me on that tho, and there are already 128904 threads about the growth rate.