1. On Bulgaria: is that you playing or the AI? This is too much for the AI, but historically Bulgaria did control almost all of this in two periods (once with Simeon and once with Ivan Asen II) Constantinople is the exception and the south borders went to Olimpia not Athens. However, it is a bit ahistoric to have this at the Ottoman invasion (maybe the Plague killed the guardsman in Constantinople or something).
2. Imperial Diet requires Catholicism.
Try another game to see if Bulgaria would get that large, I often generate Dutch starts and this is the first time I see the AI that strong. If this is happening only in rare occasions, there is no need to fix anything. Ahistoric things will be happening all the time.
Ah thank you that makes more sense
It is definitely the AI, I was playing England so I couldn't have had an impact on them
The problem is, unlike history, they are very technologically advanced, and they have numerous wonders: see this page, they have 3 of the top 5 cities, a great feat for a mod with like 30 civs!
Like others are talking about, the Ottomans are overpowered in my opinion, they only have turkey (which is fine) but are very tech advanced and are building quite a few wonders as well, following in bulgarias footsteps
My Experience playing as England (France and Burgundy are taken out because I conquered them)
Overly Powerful Civs:
Bulgaria (wayyy too powerful here)
Ottomans (historic, but still they are better tech advanced than even Germany)
Norse (Until Sweden they had about 14 cities, they completely expanded in all of Scandinavia including finland)
Kiev (weren't even dented by barbs, had a huge empire pushing on the borders of Poland, Moscovy, and Bulgaria, I'm guessing at least 15 cities)
Weak Civs
Arabia (collapsed very early - first crusade)
Genoa (as always)
Spain (Cordoba collapsed and they failed to conquer the lower half of the country)
Venezia, Hungary, Austria (mostly because of Bulgarias strength, so I'm not too worried)
New Surprises (this may just be the particular game, but civs that aren't as bad as before)
Poland, Portugal, Moscovy, Burgundy