I wonder why this comment is at the top of the HN post.
Over the years I've seen a lot of missinformation on this topic that follows pretty much this exact train of thought. Why would countries join the EU and the Euro if it didn't benefit them?
The baltics have all grown massively since the 90s when they became independent, and even though they all were on nice trajectories they still all decided to join the EU and the Euro.
Bringing up the UK as some model for all other "small" european countrie sounds odd. The UK joined the EEC specifically because it had slower economic growth than the other large EU countries.
The UK, and specifically the city of london, with its huge international financial pull has a very different place in the global economy than Bulgaria...