What split is optimal depends on what effect size you expect. If you expect a very large effect, you get more statistical confidence from allocating more subje

What split is optimal depends on what effect size you expect. If you expect a very large effect, you get more statistical confidence from allocating more subjects to the treatment group. On the other hand, if your treatment is very costly, adding subjects to the control group may be an effective way to increase study power without as much increase in study cost.