Yellow Courgette not growing

I have courgettes in the ground and some in 8" pots (no space).  The ones in pots struggle, but still produce lots of regular sized courgettes every day.  Bog standard compost with a little manure in and a weekly feed of Tomorite.   

I have 2 yellow courgettes in very large pots and they don't appear to get very big, they are producing but are very small compared to the green variety, so I think in future I will stick the green one's they grow much better.

This has also happened to my yellow courgette plant. I have 3 green and they are producing really quickly as you would expect since May. The yellow one has produced loads of little fruits the size of my little finger and have not grown on since. I went away for a week and got back to one that had grown to a normal size, the rest are exactly as I left them. They are in the ground and have been fed and watered exactly as the green ones. Very  strange!