You don't seem aware that senators are two per state (fixed in the constitution), regardless of the state population. That's often a lighting rod of non-proportional criticism; see e.g. the first article linked in What's (roughly) the smallest percentage of the US population that through its Senators has successfully blocked a piece of legislation?
senators representing about 11 percent of the population can filibuster a bill or those representing about 16 percent of the population can have a majority.
And there are those who argue the Senate [thus] is a bigger problem than the Electoral College from this disproprtionality perspective:
The Senate is a much bigger problem than the Electoral College [...]
America’s nonwhite population tends to be overwhelmingly in large or medium-sized states. To illustrate, the 10 biggest states (by 2018 Census estimates) all have nontrivial percentages of nonwhite voters, while the 10 smallest states mostly consist of rural, overwhelmingly white states.
While the Electoral College is also winner-take-all at the state level, each state’s representation is much more proportional to population. State are given electoral votes equal to their number of House members plus two senators. House seats are allocated to the states proportional to population. Only the extra two votes contribute to disproportionality [in the Electoral College].
Now it's true that actual majorities in the Senate have not reached anywhere near that 16%. By one calculation, the lowest was 47% (in 2017).
Note however that this is a measure of malapportionment among the two parties that by the design of the voting systems end up competing. In actual PR systems in Europe etc., the number of parties actually competing ends up [much] larger than two. It is however much more difficult to estimate these long-run effects as they would require counterfactual assumptions(*), but they are thought to be significant; see Duverger's law
(*) For example: how many parties would emerge e.g. if the factions in the Democrat or Republican parties would split and create and their own parties instead of competing in the primaries, which are also majoritarian. Neither party is much inclined to these kinds of splits because it would be an electoral disadvantage under the present voting systems.
As an aside, since Wikipedia doesn't have great article on Duverger’s law...
central claim is that social heterogeneity will increase the number of parties
only when the electoral system is sufficiently permissive, no existing study
actually estimates whether social heterogeneity has a statistically significant
effect when the electoral system is permissive. As a result, Duverger’s theory
has never been directly tested.
The problem like I said is what is the counterfactual scenario (i.e. what are those sources of heterogeneity that would create more parties in a more "permissive" PR system). One relatively recent (but fairly cited) paper on the topic used ethnic groups as a proxy, and showed that in countries where PR is present they get a better representation.