The flux linking the windings via the core can cause leakage inductance. To minimize the leakage inductance, the windings are placed one over the other. But this can reduce the inductance to a limit only. The effect of leakage inductance is not a great concern at low frequencies, but it is not true at all cases. While, effect of leakage inductances should be considered at high frequencies.
The core losses are dependent on max flux density which in turn is inversely proportional to Cross Sectional area of the core.
So if the C.S area is kept more the core losses will be lower. This means that in distribution transformer the ratio of Iron to Copper is higher than power transformers. So if you see a power transformer and a distribution transformer of same rating, the one of larger size is usually the distribution transformer.
Other methods to reduce leakage losses include increasing the height to width ratio of the core window.
Larger insulation between windings also results in more flux leakage. So it must be kept at optimal thickness.