"That is a little bit of a stretch" or sometimes we say "That's a bit of a stretch."
We usually say that whenever somebody exaggerates something.
For example:
Person 1 says "McDonalds hamburgers are so unhealthy! They cause cancer!"
Person 2 replies: "That's a bit of a stretch."
Person 1 is right that McDonalds hamburgers are unhealthy but the exaggeration (or the stretch) happens when he says they cause cancer.
So we say "that's a bit of a stretch" when a truth (McDonald's hamburgers are unhealthy) is exaggerated (they cause cancer).
I hope that helps. If it doesn't, let me know and I'll try to explain it better. :)