the growth power is added as a weakness that can be bought during the design of a species to stagger the full size over many levels. This is not like having a juvenile aged member of a species where they are under developed and lacking breeding maturity. These are physically mature but will continue to increase in size with level advancement, which is a kind of an simple aging mechanics. These size increases have effects on some abilities, movement, space requirements and of course food, water and construction cost when dealing with playable species.
Playable species can not have smaller than tiny nor can they have bigger than huge. Read more here.
All the Best,