Okay, I think there is some miscommunication here. What I was getting at was that the number of MEPs per inhabitant (shown on the Wikipedia page, but not yours) can vary by over an order of magnitude.

Nor was I making any statement about whether this accomplishes anything; it was merely a factual correction. Representation that differs by more than an order of magnitude cannot be called roughly proportional.

As for the material point you raise:

The EU is de facto a confederation; joining a confederation means that a country purposely accepts some duties as the result of membership, but also expects benefits. And within the EU, smaller countries generally are the bigger beneficiaries (with exceptions). Aside from the obvious (free and unencumbered trade within the EEA), some of them are:

* The EU can negotiate as an equal with world powers such as the US, Russia, or China, countries that have a habit of engaging in arm twisting to get their way vis-à-vis smaller trading parters. Being a member of the EU generally results in more equitable treatment or being able to negotiate treaties at all.

* The EU has a centralized system for the market approval of drugs and treatments; this means that smaller countries will generally not experience a delay in access to new state of the art treatments, but will get them at the same time as (say) Germany and France.

* The EU has a mutual defense clause; this is of particular interest to countries bordering on or close to Russia at the moment, especially if they aren't members of NATO (Finland and Sweden).

Obviously, this needs to come with safeguards so that smaller countries aren't run over by bigger countries:

* Voting in the Council of the EU (the upper house of the EU legislature, which represents the interests of the member states) requires a qualified majority or unanimity. While no individual member state can block ordinary legislation, for any unreasonable request there will generally be a coalition sufficient to oppose it. Certain sensitive matters also require unanimity and can be blocked by any member state.

* Principles to protect the interests of the member states are enshrined in the treaties, such as the principle of subsidiarity, the principle of proportionality, and the precautionary principle; legislation that violates these principles (or the EU Charter) can be overturned by the CJEU through judicial review. (This was how the data retention directive died, for example.)

* The European Parliament cannot propose legislation directly, but only indirectly through the European Commission; while this is considered a flaw by some, one of its purposes is to give the European Commission – composed of one commissioner each per member state – veto power over legislative proposals.

Finally, let me note that the use of "sovereignty" here is imprecise; accession to a treaty, even where you let other entities exercise sovereign powers on your behalf, does not constitute loss of sovereignty. Consider, for example, the NATO Status of Forces Agreement, under which (Article VII), the receiving state explicitly permits the sending state to exercise criminal jurisdiction over both military and civilian members of the forces of the sending state on the territory of the receiving state. Criminal justice is a sovereign power, yet nobody (outside a few crazies) assumes that SOFA rules imply a loss of sovereignty.

As Michael Dougan (Professor of European law at the University of Liverpool) put it [1]:

So let us start with a couple of myths. The first is the myth of sovereignty, there is no doubt whatsoever that the United Kingdom is a sovereign state under international law. There is no doubt whatsoever that the parliament in Westminster is the supreme law making authority in this country.

Conversely there is no doubt whatsoever that the EU is not a sovereign entity, far from being a sovereign state, it is not even a sovereign entity, it has only those powers which has been given under the EU treaties. If the UK courts sometimes give priority to EU law in the event of a conflict with domestic law, it is purely because our parliament has expressly instructed them to do so in our own legislation.

So it the UK a sovereign state? Yes.

This should not distract from the fact that de facto member states experience substantial constraints on what they can and cannot do by virtue of their membership.

[1] https://news.liverpool.ac.uk/2016/06/22/transcript-professor...