This refers to something called 'over potting', which is a plant placed in an excessively large container for its current size. Two problems may occur - in slow growing plants, the roots cannot permeate the potting mix quickly enough, and the compost therefore remains wet for long periods, stays cold, and eventually becomes sour and causes root problems. Quick growing plants may make too much leaf growth, which delays and may even prevent flowering/fruiting. Note the use of the word 'excessive' though - we're not talking about putting something with a 3 inch across rootball into a pot that's 6 inches wide - we're talking about putting that size of plant into a pot that's 10 inches or more wide.

Thanks, Bamboo. Good to know. Can't believe it has taken me all these years to try to find this out!

Fascinating :o)))
Bamboo is so good with her answers !

Well thank you, TT - I do try to be as clear as possible.

:o)))

Although it's strange that putting a plant into the ground (the size of the whole world!) doesn't bother it at all?

Lol. Volunteer :o)

.. I guess the drainage is different in the ground, to when a plant is in a pot, so the soil would not go sour etc...

Exactly - open ground is a completely different situation from compost in a pot. For one thing, the biodiversity levels are much higher and more balanced in open ground.

Very interesting, I learn something everyday. Thanks bamboo for explaining it, especially about why problems occur in pots and not in the ground, makes sense. I wondered why I lost some plants that I planted in large pots, thought I was doing them a favour... More room to spread their wings, or should I say roots? It all falls into place now. Oh! GoY is a wealth of knowledge sometimes :o)