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Is this always true for Tuberous Begonias?

Tuberous begonias will peter out as winter comes, but I cna't say the blooms get smaller.

Cut the stems back to allow the tuber to rest. I hope you have not been watering this. It should be stored in a cool (you have the right temp) dark and dry place. I put mine in the basement (Zone 5) and pack in sawdust.
. My first year tuberous begonias, that I have grown from seed, I leave in their containers and allow to go dormant in Nov./Dec.. Older tubers are remove from the pot and cleaned and stored in the sawdust. I use blueberry containers. I remove all stems as they wither before putting them in the containers.

In March, I will move the one year old plants to a warm room with no direct light, and water them. If they show signs of life in a couple of weeks, I have succeeded. I will them remove most of the soil and repot.

The clean tubers get the same treatment. I lay them out in a bed of sphagnum peat moss (the stringy stuff) without covering them and with the hollow, flowering side up. When they show signs of rooting (white, fleshy roots on shoulders and bottom), I will pot them up. From there, they need strong light to keep them from getting leggy. I use flourescent lights for 16 hours a day.

The reason you should leave young plants in the dirt is that the tubers tend to be so small that they survive better if they are not disturbed in the same fashion as mature tubers.

Thank you very much for your information. I agree with you. After I moved them inside my house in November, they looked tired. Remember I bought them as rooted tuber-less stems. I dared not let them go into dormancy. They kept going in my cold and bright window over the winter. I maintain minimum water. The one with the entire stem disintegrated, it had a small tuber and a new shoot already. Although the new shoot is very bristle and I accidentally smashed it off. The other two tall stems are largely leafless, although there is one or two new healthy leaves started from the bottom already. I did not cut the stem back because I thought these stems contain "energy"  like you should not cut back dying leaves of tulips. I was waiting for them to dry up  they never did. The leaf-less stems are still reddish green with the glow of life now in February. They never rest since last March. I hope they come back despite the fact that they never had a good sleep. I will definitely follow your practice next year. When you use sawdust to store the tubers, do you use damp sawdust? When you move the dormant tubers into a warm room in the spring, what is the temperature? When you say "strong light", I assume you do not mean noon or afternoon burning sun of the summer? I love these very much. They had gigantic salmon-red flowers with bright yellow centers. How beautiful!

I measured yesterday. These are 10" tall. They never die off completely since last March.

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... and my cyclamen:

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Just an update, by yesterday night, both stems fell over. Hehe. But there are still small leaves at the bottom.