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There's nothing wrong with either of them (I hope!) but I was just curious as to why the cutting on the left, which was made from the one on the right a couple of years ago, has grown so differently? The cutting appears to have opened up more, and I don't think the original needs re-potting unless someone disagrees? Just curious

The cutting is getting more water and maybe more nutrients, relative to the amount of plant, than the big one. Perhaps and indication that the big one could handle a bigger pot, or maybe just some fresh soil. The cutting may also have been getting less sun, which would mean less spines and a fatter plant.

Cheers Iann, maybe it's time to re-pot the parent! I'll give it a try. Do you think it still looks healthy though?
Mike.

They both look healthy from what I can see.

I'm just looking more carefully and think the large plant is in a clay pot and the small one in plastic? In my conditions, clay pots dry out 5-10 times faster than plastic ones. Your conditions are perhaps different, but I bet that clay pot is dry well before the plastic one after you water them.

I consider any cactus that is hanging over the side of the pot as a candidate for repotting. Occasionally I get it out and the roots are not up for a bigger pot, but usually they are.

I went for mainly clay pots because quite often, especially when the soil is dry, something that size can be a bit top-heavy! I've read many debates on clay or plastic, with no clear cut winner on either side. Looks wise I think clay looks nice, and holds the heat better? But you're probably right, clay will dry out quicker. I'll certainly check the root system tomorrow, I re-potted two of my Mammillarias a few weeks back and they were in pots about 6 inches across, they were root-bound so definitely needed re-doing.
Cheers as always for you advice Iann,
Mike.

Could also be the one on the right gets more sun. I know cacti that I have in the 'hot' part of my GH vs under the shade-cloth will look the same as your picture when side-by-side. Usually cacti will grow more compact and spiny in higher sun - the spines act as sunscreen. Compact and spiny is usually the 'habitat' look, so IMO you should try to find conditions to make the one on the left look like the one on the right.