Pictures would be helpful to clarify what you're concerned about. I grow my tomatoes and annuals in 4-cell packs and even then the tomatoes need moving up to a much larger size in a few weeks. They are really fast growers.

I'd imagine the OP means something like this:

which is how I buy my seedlings when not growing from seed, which would seem to contradict all the advice about repotting early when the plant is a lot smaller than this.

I worked at a garden place that ordered their plants. And from what I heard there- nothing special to save space really. Growers start out seeds by the flat then transfer the seedlings into those cell packs. And then grow out in the cell packs and no more potting up. Saves them on soil, pots, and labor costs and helps make the plants "look big" because there's a lot growing out of the top. Makes customers feel like they are getting a good deal because the plants look big in the cells. They feel like they aren't getting "their money's worth" if the plant looks small in the pot, regardless of what the root formation may be in the pot.

For example: a 4" tall tomato in a 6 cell pack looks big by comparison to if that plant were in a 3x3 pot. And the 6 cell pack can take as much soil as one 3x3 pot. A lot of customers like this because the cells get really packed with roots and they feel that means the plants are ripe to get planted. Then the "step ups" are 4 cell (uncommon around here), which customers feel they get bigger/better plants, then up to the 3x3 pots, which can be sold as premium plants... It's more about cost effectiveness, ease of transport, and customer marketing.

It's why sometimes those cell packs are overly packed with roots- they probably should have had a potting up, and a lot of home growers would have to encourage additional root development.

I was hoping to see the OP's own plants to see what might be his/her specific issue.

That's a healthy looking plant, and I agree it could probably use a potting up. But commercial growers don't look at the root masses in the pots- they look at the plants top part. At that plants height it looks like it would be ready to ship out to the garden centers and their customers.

Most plants you buy are root bound and need to have their roots loosened and repotted or planted out. Unlike gardeners growers are not so concerned about roots as they are about top growth. Have you ever noticed that at the end of season plants look awful even though they should be large and thriving. It's because they are root bound. If I bought that 4 pack of tomatoes above and it was too early to plant it outside I'd loosen the roots, pick off all the bottom leaves and sink each one down a couple of inches in separate 6" pots.