Same thing happened to me last year. I had a few small cucumbers (it was a pickling variety) and I was a bit sad I wasn't getting much out of my plant. One day, I moved the plant around a bit and wow, there was a ginormous cuke underneath the leaves. Like 3 inches in circumference and about 8-9 inches long. I was worried as I heard they taste bitter if they are overgrown, but I picked it and made a cucumber salad (with one cucumber, LOL) and it was the sweetest cucumber I've ever tasted. After I picked that one it produced quite a few more. There wasn't enough for me to do any pickling, so I let them all grow huge and they were all delicious, LOL. I planted the same variety again this year and crossing my fingers, since I put them in the garden this year instead of a container...hoping they produce the same way 
Edit history
= new reply since forum marked as read