An analogy:
Imagine a subdivision with 10 houses. Nine of those houses have a single person living in them. One house has a family of 15. When we go to vote on new rules in the subdivision, the unit of voting is the house. We don't let the family of 15 control the entire block.
It doesn't matter that this means those 15 people get the same vote as all the single people, because that's not the voting unit. The logical unit is the house. They can vote within their house about policy within the house, but they vote as a group on neighborhood policy.
This is federalism.
The unit is the state. Missourians combine to form a vote, just as Californians combine to form a vote, and so on. Individuals within the state vote individually on issues within the state. But they don't vote as individuals on national policy. Their votes are aggregated by the state in which they reside.
If this seems odd, it's only because modern Americans have largely stopped thinking of the United States as, well, a set of united states. But that's exactly what they are.