It sounds like you're inadvertently benefiting from an advanced concept called "compression".
A yellow iron-belt can send at most, 15-iron per second, towards the factory. The player can spend this on 3-steel per second, 15-green circuits per second, 7.5 gears per second, or any such combination of intermediate materials.
However, "steel" has an interesting property, every "1 steel" you send down the factory is equivalent to sending 5-iron down to the factory.
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This means that your singular steel belt sending only a partial belt (maybe 6-steel/second, which is only 40% capacity) is "equivalent" to sending 2-iron belts (30-iron per second).
"Compression" means that you only have to think about running 40% belt of steel, rather than 2x 100% belts of iron. This takes up 1/2 the space and is far simpler in the overall picture, and even leaves room on the belt for up to 2.5x expansion later. (Your 40% yellow belt can "expand" to 100% capacity in the long-game, ultimately reaching the same throughput as 5x yellow belts of iron at 100% capacity).
Playing with "compression" can lead to simplified designs. Similarly, green-circuits are great at compression (1-iron + 1.5 copper per circuit), as are gears (1-gear == 2-iron). But steel is among the best (5-iron per steel).
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If you work at the "steel" level and plan around a "future steel belt", you end up benefiting from compression and leading yourself towards simpler designs.
But that's an advanced concept. This isn't something I'd expect a beginner or intermediate Factorio player to get. The beginner/intermediate player would try to brute force the additional iron-lines using iron plates directly, and have a difficult job of doing so. It takes multiple playthroughs before you realize that belting-steel has benefits over belting-iron from a factory throughput perspective.
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The recommendation I put above, to belt your science to new locations, is the ultimate answer to this compression. A blue-science pack consists of 1.5x adv. circuit + 1x engine + .5x sulfur
That is to say: one blue-science pack is 7.5 copper + 12 iron + 3 plastic + 1/2 sulfur, or roughly 23-resources per blue-science pack. That's a compression ratio of 23-to-1, superior to steel.
This makes "belting" blue science around a relatively efficient endeavor from a human-labor perspective. (True: the factory has to work 23x harder to "fill the belt", but if we're talking about "eventual consistency" measured in the scope of hours, its not really that big of a deal to wait for the belt of science-packs to fill up to its steady state level.
And by "wait", I mean, "play the other elements of factorio while waiting". I know its going to take 3 or 4 hours to happen, but I know it will happen with 100% certainty because I have confidence in the design. So I do it, and move on to base expansion / building more train outposts or other manually-intensive jobs.
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In any case, "just expand steel" turns out to be one of the easier tasks. I don't know if you knew steel was specifically a high-compression item worthy of attention like this, but... that's an explicit strategy. One that took me many, many playthroughs to understand and take advantage of.