When leaves and stems wilt it is because they are not getting enough water. Obviously, that can happen if the watering is neglected and the soil around the roots is allowed to become too dry. However, wilting can also occur for the opposite reason. If soil is kept too moist for too long and roots are deprived of oxygen, which they need as much as water, then the roots start to die and are no longer able to absorb water for the stems and foliage.
Judging from the size of the pot and the abundance of excess soil, my educated guess is that the roots of your Begonia are dying from suffocation or lack of oxygen. At this point, I don't recommend undoing the repotting and downsizing. However, I do suggest that you remove all of the execs soil you added to the top of the original rootball. That soil is extraneous and is keeping the soil in the root zone from drying out appropriately. Following that, allow the top inch of soil around the roots to dry out before adding just enough water so that it reaches that level of dryness again in about a week or less.
Most flowering plants flower best when they are moderately potbound. That is another reason not to repot unnecessarily.
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