How does Goku's hair grow longer when he becomes Super Saiyan 3? TIL Super Saiyan 2 Goku's hair is the same length of his normal hair, but when he transforms into SSJ3, his hair starts growing and becomes pretty long. 

Even if the hair grows while in SSJ3, it should not shrink back when he's returns to normal. The growth could be the result of the pressure exerted from his body, but shouldn't shrink back when he's normal.

Is there an explanation for that or is it just the way it is?
Before we begin an answer:

I prefer honestly saying that there is no answer for this (or I personally never saw something that answers that on the web), but we can use some deductions from what we know and experienced from the long series of the anime.

As we already know from Dragon Ball, the trademark characteristic of the transformation is the user's hair, it goes golden yellow at the Super Saiyan 2 level as a representation of achieving a higher level of energy.

I guess you could also ask:

why does his hair goes golden yellow at the first place ?

For me I know no reason of getting hair color changed when rising the energetic level, but by assuming that saiyans are a different race than humans, we could explain it as a result of the rise in Ki consumption, for example in SS3 : 


  The rigid hair of the Super Saiyan 2 state becomes flowing and smooth again, and grows down to or sometimes passes the user's waist. The eyebrows vanish completely, making the forehead and eye ridges appear larger and revealing a more prominent brow ridge. A small increase in muscle mass is evident, and muscle tone is sharply defined. Energy radiation is so great that the aura pulses at an extremely high frequency, almost to the point where it seems static; the sound of the aura is also pitched even higher than that of Super Saiyan 2. Bio-electricity, like in the Super Saiyan 2 form, is again constant, and may reach further outward from the body than before. The Saiyan's voice may become slightly deeper, though this is obviously a feature only found in the anime. If the user of the Super Saiyan 3 form has a tail, it turns yellowish gold. 


Source

So as a personal explanation:

I can say that just like when you eat a lot you get fat (as a human), when you open your body for a perfect Ki consumption (as in SS3 level) your hair will grow as long golden yellow (as a Saiyan).

And as a remark:

The body must change the morphological shape to achieve a higher efficiency in using the current level of energy for combat (for example largest muscles in SS3, tallest in SS4 .. etc), and as one can deduce: hair has no effect on the combat course, but personally again, I think the complete opposite: the hair style may cause more effect than muscles, it have an impact on the moral and psychic state of the enemy, to make it even clear:

Just like the lion is scary with large hair, so is the saiyan in SS3, he looks stronger sometimes more than what he really is. Unlike what was the small evil Boo, his physical appearance didn't show his strength therefore it had no impact on his enemies (Goku and Vegeta).