Small roots and a big plant are often indicators of too much water and fertiliser. It is not uncommon to buy plants that look large and robust, but they roots being small and tiny. In greenhouses where plants are forced to grow faster with extra water, heat and fertiliser this is often the case.
I don't know what size pot it was in before you took it out. But, I would assume it is probably the appropriate size for this plant. Make sure when you water, you wait until the soil is dry at least 3-5cm (1-2") of depth. If you water too soon the roots have no reason to travel into the other parts of the pot.
I know your plant has been wilting, so thinking about adding less water is counter-intuitive, but not for the roots. Also avoid fertiliser for the time being the plant is nice and green it does not need any more food.
Normally I would say fertilise every 3-4 weeks during the growing season, but for you I want you to avoid fertliser completely while you trying to promote root growth, especially fertiliser high in Nitrogen. If during the growing season you see the plants leaves turning from dark green to a paler green or yellow, you can then add some food but for now you want to focus on promoting root growth.
You can add some bone meal to the soil when you put it back in the pot. This natural added phosphorous will help stimulate root growth.