You can grow from 7-B to High 7-A, 6-C or 5-C without exponential growth. Exponential growth is something like Saitama's: if at first you have a power of 1, after 1 punch you'd have a power of 2, then a power of 6, then a power of 24... (the gap is increasing each time, as shown in the fight)
Garou growth (before Cosmic form) is something more like: if his power at first is 1, after some time it's 2, then 4, then 8... (the power increases, but does not make the gap between each power increase higher)