Static Strength

Can anybody help me with a physiology question?

When you do Isotonic muscle contractions using weights you pump fluid into the muscle and recruit dormant satellite cells- hence the increase in muscle size.

However, what happens when you do static contractions? ie. Handstand, Plank, Peacock
Obviously your strength improves but your size does not- I have seen rake-thin yoga teachers who can do handstands for 10 mins+.
How does the muscle get stronger, what actually happens?

I ask bcos I know a lot of people dont actually want to get big but do want increased strength and improved posture and balance etc.