I get more questions about génoise than about any other type of cake. I understand that. Génoise batter can be tricky stuff. Oh not because the technique is difficult to master. The steps are mostly pretty easy. The danger lies in over-whipping.
It took me years of on-again, off-again génoise making to understand that the degree of whipping and the height of the rise only correlate up to a point. Once you pass it, more whipping begets less volume, even a complete and utter failure (fall). Why?
The reason is the size of the bubbles. Lots and lots of whipping creates larger and larger bubbles. When the cake is put into the oven, those bubbles increase in size. The batter around the bubbles stretches and stretches trying accommodate the volume, but eventually the structure becomes so delicate it can’t support the weight of the batter above it. The bubble pops. That leads to a domino-like effect of more and more bubbles popping, and the cake falls.
What to do? Be careful not to whip the eggs and sugar to the point that ribbons begin to fall from the beater (or whip). If that happens (and it will from time to time), just proceed according to instructions. When you get to the form-filling stage, just knock your batter-filled form on the counter a few times. That will pop most of the really big bubbles and allow the batter to bake up to a reasonable volume.