Looks like a lot of them are planted on top of each other, in which case, competition is a factor. Hard to see, but are any growing willowy that have their own space?

Were all the onions shown - large and small - all planted at the same time? The small wispy ones look like very young onions... to me.

The tiny ones were from seed, big ones were regrow. May be time to thin out the new growth?

Scallions, I presume. They have been outside all of the time? They don't grow very strong inside.

They need sun, not mostly shade. Seedlings don't like it too hot, I assume that weather is pretty mellow in Jan/ Feb. I don't know if the soil is the a problem, potting soil isn't all great, but hard to think it is that bad.

Scallions actually ARE green onions, harvested when immature. The OP is wondering why they aren't maturing.

True, but many folks including myself plant non bulbing onions for scallions. Maturity is not a question at this point. Trigger daylight hours for all types of onions are a ways off. Healthy growth is important at this stage, dark green leaves, erect and healthy looking.

There is an allium forum, or tag (@alliums?) here, and I frequent it because other than garlic, shallots, and chives, they seem to tricky to me. Many variables, at least for me.

I dont think the regrows will bulb, but you folks say you can use them as green onions again. The onions from seed are supposed to be cut back now and again so they don't flop, and thinned before they go to their final resting spot. Size and timing of that, I am not certain of.

Climate and is also a factor, so I grow intermediate day cippolini that I that don't very big, from fall through early summer here in N Cal. Bay area.

Here are mine today; I planted them as seedlings in November.

Don't take my word though, as I have had rare success at getting the bulbs I'm looking for. I'm going to be more diligent about fertilizing this spring.

You can thin and replant the ones you pulled, I have. They need more sun. As they grow more roots, they will thicken up and straighten. Space 2 to 4 inches apart.

https://www.bonappetit.com/story/are-scallions-and-green-onions-the-same-thing

Allium cepa is bulbing onions, our normal kitchen onions. You can use the leaves as scallions, or even plant them too crowded together intentionally for the purpose of scallions. Allium fistulosa or "bunching onions" are perennial and don't bulb up. They are only used for scallions.

Regularly have Allium fistulosa, or bunching onions, in pots do this when it's a combination of cool temps, low sunlight, and heavy frequent rains. Basically, winter here. Could post a pic of all my last spring seeded pots in a part shade spot doing this right now. Pots also don't usually drain as well, especially if set up for the other nine months of the year when it's hot and dry. Sometimes they can go so far as to root rot before spring and the accompanying hot weather comes. This can also happen if you have never given them something nitrate like, but as green as they are I'm assuming you did that.

My fall and early winter seeded babies are in a bright winter sun spot so aren't looking limp, but are almost as thickly crowded in their pots. They can't stay there, here, during summer. Mine need to be moved to shade for summers.

I don't initially space the seeds or replants of roots that I intend to pull for the whole tops and stalk, more than 1/2 inch at most, unless I want thick stalks. Or when I'm letting them go to seed. They produce more seeds for me, in a pot, if they have more space to thicken and clump out. For a full clump I've allowed 6 or more inches between plants.

If you have any other places to stick them (and they are bunching onions) I'd thin those center new from seed babies soon into something that drains well during the winter as backup clump starters. Or move the pot into a place where the sunlight hits during these months. In the meantime, cut and use the regrowth larger ones at the bent part in the bending shoots to try to discourage them making seed heads so early and so small, unless you are out of seeds anyway.

Just be careful to leave as many shoots as possible. If these are your only or first starts, I wouldn't harvest any shoots at all until they divided, start to go to seed, or formed a good clump anyway. So you have a seed production base of plants. We use 10 or so a day, often as chive substitutes because chives just don't want to grow here unless it's cool. Sometimes even most of the white stalk is used, so I'm planting out one year's regrowth roots and seeds pretty often. Oh, seed viability has lasted a nice amount of time for me. Just set out a batch two weeks ago grown from a set of seeds saved July of 2017, which had gotten lost in a back drawer.

If they are Allium cepa being grown for green onions as immature before bulbing or not allowed bulbing of onions, then the above advice from other posts seem better. I've never grown bulbing onions specifically for scallions/green onions. Or in pots.