What does this mean? I feel like it's an idiom of some kind.
No, not an idiom.
I wish it was a sentence and not just an expression.
"There are big cathedrals and small churches in Ukraine" - "В Україні є великі собори та маленькі церкви"
Ukraine has many large cathedrals and many small churches.
In Ukrainian there is also word "Костел". "Cerkva" is only orthodox (and greek-catholic) church.
Костел is a loanword from Polish.