Leeks

Mine are rarely bigger than a darning needle when they go out into the ground and are always fine, easily wrist size by Christmas (even though they often disappear from sight into the planting hole).

Hi PP

Is the planting hole massive in diameter compared to the leeks?

Yup!
I use a standard dibber,  30cm deep, 4-5cm diameter, and the leeklings drop out of sight (each one gets a little compost and some water dropped down the hole after it). The blackbirds leave them alone, but the young plants are also protected from the heat - often low 30s by June - while the roots get established. After about 10 to 14 days the plants appear above the edge of the hole and happily grow away.
By December most will have filled the hole