Say you've just read an online discussion post and want to comment on the fact that the writer has jumped from point A to point B without attempting to convince of the validity of the connection. What could you write on that would be more refined than "that's a bit of a stretch"? I should mention that I don't have continued interest in understanding their reasoning and am looking for no more than another way to say "that's a bit of a stretch."
I might format it like a question:
Jumping from A to B is a big leap of logic; care to explain your reasoning?
You didn't explain the connection between A and B very well; care to explain your reasoning?
The latter fragment, care to explain your reasoning? could be omitted at your discretion.
The logical jump between A and B is a bit of a stretch...
Would be perfectly reasonable -- It's not too informal.
Optionally, if bit of feels too informal,
The logical jump between A and B is quite a stretch...
Formatting it as a question like this isn't necessarily the only way to do it, though. Use your discretion.