Cucumbers going mad!

My cucumbers aretaking over the greenhouse. I only put three in a grow bag and they've turned into triffids!

However, i'm not getting any big cucumbers developing - have loads of flowers with little cucumbers but none are getting bigger. Do I need to chop them back or pinch something out?

   I think they need feeding - three in a single growbag is a lot.  They look healthy though!    One of the cucumbers we've grown this year set a lot of fruit, which all started going yellow when it was about an inch long.  Realised this was because it had several fully-grown cucumbers on it, so it thought it had done its job!  Took the full-sized ones off, along with all the little yellow ones, fed it and it started flowering again.

If yours have produced loads of small fruits, you could try removing some of them (perhaps on one plant as an experiment) and see if the others start growing.

Thanks Slowcompost, this has helped me with my cukes which are going mad too!