I had some difficulty with that as well; I can't speak authoritatively but here's what I was told =)
"tha luch beag" -> "a mouse is small", a complete sentence about a mouse
"tha luch bheag" -> "a small mouse is ", a sentence fragment that needs more to be correct because the small mouse is
It's the difference between saying "my foot is sore" and "my sore foot is ".
Does that help?