Can replace 50 percent of flour with cornstarch, thereby lowering the overall percentage of protein. Sponge cake structure comes primarily from egg protein reinforce by starch from the flour. If you completely replace with starch, the texture will not be as light or resilient.
The most important ingredient is egg, therefore, the way it is beaten, temperature and the manner of adding other ingredients to egg is crucial.
Too much volume is not desirable either because if there is more volume than the structure can support, the genoise will collapse