conversion

Whilst looking through baking websites I came across this conversion from yeast to sourdough which puzzled me. It said for every teaspoonful of dried yeast remove 62 grams of both flour and water and replace it with 124g of starter at 100%.

That it itself didn't sound too bad but when i tried to convert one of Paul Hollywood's recipes I stopped to ponder as it calls for 5 teaspoons of yeast. That would mean exchanging 310gms of flour and 310 gms of water for 620 gms of starter which seemed a huge amount to me??

Comments??