I checked on that yellow spined carmenae and it came to me 4 years ago in a 4 inch pot. So not exactly the fastest plant (now nowhere near close to filling an 8 inch pot), but pretty nice form and color. I have a second one also with yellow spines, have not seen lots of other colors. I like the branching (clumping) behavior. I like the compactness.
My observation from those years of culture is the plant is not a deep drinker. Soil dries out much slower than usual. I water half as often. You could correct by putting more rock in the mix, for the same effect. Other plants in the same category: M. geminispina, nejapensis (when not underpotted), 'Un Pico', poselgeri, sometimes plumosa. I keep them in a group, so I don't overwater. Or I put them in an unglazed clay pot with the maximum exposure.
This information about the slow-sipping behavior of these plants comes from the state they were in at the last repotting (timed a day or two before the typical watering interval, which is every week year round). Observations about the soil moisture dictate how I water some plants differently.
C. (M.) dioica on the patio was doing some crazy reproductive behavior when I went out to water this morning.
Here is a different stem, picture taken a few hours later, when I was a little more awake.
It is highly unusual for the flowers on this cactus to be open this wide. Today was a very warm, sunny, low-humidity day. I think the last part is what matters most as far as at the flowers opening wide. As for why there are so many flowers on that stem (outlier behavior), I would say midwinter, 10 inch pot, it's time to make fruits and seed and babies!
Picture from 4 years ago:
Oh how time flies.
My watering days start early, around dawn. It's a little rough in the beginning but I structure things so that part does not require excessive awakeness.

Dawn over the ocean on an overcast day (yesterday, which was another watering day):
Recently installed Didierea (center) gonna photo bomb this image more or less forever, I hope.