How do you prevent your spongecake from shrinking radially? I mean I have a smaller mold so I just put the cake there to do away with any gaps (I then pour a liquid over it that cools down to become mousse of sort, that's why I don't want gaps). But what if I hadn't? Did I do something wrong? 30 g of flour per yolk (4 in total) + half a cup of milk + 40 g of melted butter, I then mixed it with whites whipped with sugar and honey. 150°C, 45-50 minutes, with a basin with water on the bottom of the oven  
The issue is caused by heat distribution from the pan and oven to the centre of the cake compared to the outside edges. The edges have much more surface contact with the pan to cook and set sooner, and as they pull away from the pan more surface area is exposed to oven air for more moisture loss while you wait for the centre to set. Most sponge cakes I've seen reduce shrinkage by maximizing surface contact with the pan, minimizing surface area open to the oven, and minimizing distance from edge to centre - like with bundt pans. cube loafs, and muffin cake styles. If you need to maintain the radius, a bundt pan works best and you can fill the hole with other fillings or a smaller cake you bake at the same time. If you can only use round cake pans, try one or a combination of these:  Bake part way with the top edges covered with parchment, with the centre still exposed. You'd need to cut a circular piece for this to work, and the trapped steam at the edges will affect the rate of browning and texture. Add lots of steam while baking, more than currently. Preheat the oven with a dry spare sheet pan at the bottom, pour boiling water on the pan at the start and whenever it dries for faster steam generation than a standing pan of water. The rate of drying may slow if the oven air is closer to full saturation, though this may have random effects on cook time - increase from heat loss with opening, decrease from more efficient heat transfer to the surface with steam. Increase batter adhesion to the pan sides. Dust the sides with flour instead of greasing, or other methods to prevent the cake from pulling away until you separate it after baking. If the rate of evaporation is still high, you will likely develop cracks on top as it shrinks while sticking - though this may be easier to cover up with your mousse topping. Bake multiple thinner layers and stack them. The goal is for the centre to set sooner with less mass to heat and less depth for heat penetration.  