Answer: It is really difficult to keep woody plants in containers long term under home conditions. At the nurseries where they are raised prior to sale, they are carefully fed and watered and up-potted with constant testing and fine tuning of the soil and fertility along with monitoring of their growth. This is just not practical to do at home. So most often these are grown as the equivalent of annuals and are replaced each year -- very few survive their first winter let alone two.
Certainly a too small container will constrict the roots and limit take-up of water and nutrients, eventually stunting the plant. Generally speaking these are plants that should be planted in the ground, in part to give them space for their roots. (Roots usually take the same volume as the branch structure and would extend outward well beyond the branches, to give you an idea of the cramping that happens in a container.)
Sometimes root pruning along with careful branch pruning can be done to limit the size of the plant and maintain it in the container (similar to growing a bonsai), other times you really do need to increase the container size. Depending on the specific cultivars you have it may be possible to keep them in the containers without growing all too much. For example, a very small and slow growing Japanese maple cultivar will do better long term than one of those that ultimately naturally grows larger -- some are quite fast growers. Similarly, the naturally small and slow growing dwarf Alberta spruce will do better than the much larger species.
So there is no easy answer to your question. I would suggest you work with your local county extension to try to evaluate their overall health and judge whether or not your trees can be kept if moved to larger containers at this point, or not. They may also be able to help you determine which type of soil mix to use, along with a fertilization plan to keep the plants healthy without encouraging excess growth. You will also need to pay special attention to watering and provide them a sheltered location for the winter.
I'm sorry I can't be more specific for you long distance, there are just too many variables involved. Good luck with your trees!
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