The phrase "gradual increase" means that something is getting bigger or growing, but at a slow and steady pace over time.
Sentences with «gradual increase»
A five or ten minute walk two or three times a day, with gradual increases in frequency and duration, is an easy way to build up activity levels. (wallerwellness.com)
All the cheesy metaphors aside, it is the principle of gradual increase of resistance. (roughstrength.com)
Most people that I have seen have done this and found that there is no problem with gradual increases in fat. (whatthefatbook.com)