What does internal growth mean? When I was growing up, my parents used to make little marks on the wall as they measured how I grew. I would look back after a few years and say 'wow, I only came up to there!?" But when it comes to internal growth, there are no marks and no wall and no getting taller. If your doctor tells you that you have an internal growth, its probably not a good sign. So what are we talking about!?
One way of speaking about internality in Lashon Hakodesh is the word בית as in the phrase מבית שמות כה יא) ומחוץ), inside and out. It means both "home" and "inside." To understand their interrelation, see the gemara Berachos 11a which discusses the verse in Shema ודברת בם בשבתך בביתך ובלכתך בדרך (and speak them while you're sitting in your house and you're going on the way). The Rabbis learn from the word בביתך that the Torah only obligates you in as much as you are free, so to speak, in your own domain. In other words, בביתך, in your home, means a place that is there for you to be under you're own jurisdiction, as opposed to being obligated by the commands of the King, under the dominion of the "Kingdom of Heaven." I think this is what we mean when we say "internal." It is a place that we perceive as being personal, not necessarily subject to any external forces or truths - a private reality that may or may not correspond to a reality outside of us.
A home is a place hidden from the outside, where you establish your own norms and expectations. The Rabbis (Berachos 4b) built a "fence" by obligating us to say the Shema before midnight out of concern we might say 'I'll just go home, eat a little bit, drink a little bit, sleep a little bit,' and then fall asleep until the next morning." They saw that home is a place where a person doesn't feel the imposition of heavenly obligation, in particular the obligation to accept upon oneself the Higher Reality. (Also, see the Remah in the begging of Shulchan Aruch, that says the way a person behaves in his home is not like he behaves before a king)
Internal growth is how to integrate the Higher Reality into "home." The Alter of Kelm says there are two ways of conquering the evil inclination - by subduing it from the outside, meaning controlling it with the intellect, and destroying it, meaning making your lower self desire the good from within itself. The Alter says that is what it means that on Purim the Jews re-accepted the Torah out of love. Seeing the way Hashem watched over the Jews and planned their salvation in a hidden way (without defying nature) inspired the Jews to accept the Torah in the hidden part of themselves. Much like a persons activities at home, they accepted to keep the Torah willingly and not only out of a sense of obligation.
Rav Elyahu Dessler says every person's free will is relative. While one persons challenge might be not to steal and murder, another's challenge may be to come to morning prayers a little earlier and give a little more of his time to the needy, and it would never cross his mind to hurt someone. The goal is always to move the 'battle-front" a little further, so that what was once an impossible sacrifice becomes a matter of course. This is part of the same idea. A person can see the realm of theft or even murder as a place that is not subject to objective truth, so that his personal feelings decide whether to take another's money or even his life. That can also be his "home." On the other hand, Avraham Avinu's home, his tent, was open on all four sides to receive guests. He dedicated his private domain to the world at large entirely!