The penis has little choice in the matter, if you get longer when erect you will eventually become longer soft and the same thing applies for girth. In both cases there may be a period of time that one progresses faster than the other but over time both will increase in size. Flaccidity, usually, will not give a definitive size as many factors are in play when soft but relative to your size now your soft penis will be larger as you grow in erect size.