If something is a stretch, usually means that it is not very believable or that something doesn't fit. He kept saying he's only had one drink, but the way he was behaving made it quite a stretch. He has been compared to Pele in football, but I'd say that's a bit of a stretch. Likewise, 'not too big a stretch' means that the story is believable or that an analogy or comparison fits. "To say that Leed's United is one of the best teams in the world is not too big a stretch." You could also say 'arguably". "Leed's United is arguably one of the best teams in the world." It means we might not be able to prove it definitely, but it is still believable.

I think you mean "It's not too big OF a stretch." It means something is not very likely to happen, but still has a decent chance. If a sports team has about a 25% percent chance of winning its next 3 games in a row, it's not likely it will happen, but it's not too big of a stretch to say it will win all these games.