> I downvoted it because the comment has no substance besides "EU bad". It adds nothing of value to the discussion.
A pretty unfair characterization. I found value in of itself, even if that value was merely "this opinion exists"
> Explain why the EU is an "oversized bureaucracy".
In order to explain why you must define how big a bureaucracy must be in order to be considered oversized. Unless two parties can agree on some ground level items, you can't hope to have a productive conversation in good faith.
I am assuming you are at least starting from the position that the EU is indeed a bureaucracy? If not, then allow me to at least explain why I believe it to be so: the EU is very formal and rigid in it's mode of action. It loves process, and it loves sticking to it. It's full of, ironically, idealistic technocrats.
It is, in theory, "oversized" because of the mere principle that bureaucratic processes can have sweeping changes that affect every member. (as an aside: often times without those members having been able to sufficiently voice their opinion, concerns, or have proper representation, thus also making it undemocratic in my opinion. Representation is democratically elected, but it is unbalanced, and all votes don't have equal influence)
Many political thinkers believe the EU should have never expanded beyond Western and Northern Europe, because, among numerous other reasons, the economies and cultures were too different. Indeed, the members that typically have the most grievances with the EU body are those neglected eastern European states, or those of the Mediterranean, namely Greece. In fact one could argue the Euro in of itself brought many of Greece's financial problems onto itself, much less the unfair-to-Greece trade mandates of the EU, but that's a story for another day.
All of this to say, there is plenty of reasoning to argue either way that the EU is "too big" or "just right", but ultimately you have to define what "too big" is, and agree on what you believe the function of governments should be.