It may either increase or decrease the size of your digimon among the minimum-maximum range of its hatch fase.
You may only use it on digimons that are either:
4/5 hatch phase digimon - Which can go from 110%-130%
5/5 hatch phase digimon - Which can go from 119%-130%
For example: Using a Fruit on a digimon that's 5/5 with 125%, may go up by 2% to 127%, all the way to 130% if your really lucky, or it can also lower it by as much as back to 121%.
Hope this helps!