Spring Onion advice ?

[Tarragon2017]

Dear Fellow Gardeners

I was wondering if you could help me with Spring Onions

My Spring Onions always end up rather thin, I have tried a few in a large pots and in smallish pot 3 Litres. Alas sadly the result is the same.

Are there any tips to get nice and fat Spring Onions

I presume the period of growth is a month after initial germination?

I look forward to receiving your sage advices

Warmest Regards and Thank you