First of all you need a very fertile soil, dig in well rotted muck a few weeks before planting or plant in soil that has been well mucked for a previous crop. How far apart do you plant your little leeks? The closer the planting the smaller the leeks. I too grow Musselburgh and plant them about eight inches apart, some do six inches. Water well until the soil is level, i.e. don't fill in the planting hole, just water and the soil will wash down and fill the hole.
Good quality seed or plants helps too.
My leeks are as they should be, short and fat ( a bit like me
)
Also, they do better in a clay soil rather than a light, sandy one, I think.
On the 8th March I am going to an evening with Monty Don, a Christmas gift from my younger son and if anyone knows his leeks it's him! I shall pick his brains on your behalf, so if you let me know where you live, soil type etc. I may have some expert knowledge for you