How to choose outsourcing company/subcontractor?

I am a self-employed software developer and I'd like to grow my business by outsourcing some of the development work.

I am located in Europe and feel that there are lots of skilled developers in Eastern Europe.

How would you go about choosing an company to outsource to? In this case I have no recommendations to start from and there are several companies that look good to me.

Having a consultant background myself, I am not sure if I would prefer a larger company: Of course everything looks a bit more professional, they have great references, everything is polished. But I know that in practice many of these companies may simply use junior developers internally, they may care more about hiring new people to grow than about quality, and you might end up getting not-so-great results.

Whereas I have very good experience using a small company as a sub-contractor of really, really good developers. But they're too expensive (I hardly make any money using them). So maybe go for a smaller company from Eastern Europe?

I'm really quite new to this, so it would be interesting to hear some feedback from people with experience.

Thanks!