There is a basic difference in the results of winter pruning and summer pruning of apple trees. In autumn, the apple tree draws down its energy supply from the smaller branches after leaf fall, and stores it in the main trunk and root system. It is important to appreciate the significance of this, for it has implications for the re-growth of new branches, growths in the spring if winter pruning of apple trees takes place.
Not too Much
If you cut out large portions of the tree in the winter months, you will in effect not be affecting the apple tree's potential for growth the following year - the energy is stored away from your pruning cuts, in the main trunk and larger branches.
The following spring, the apple trees energy supply - which is geared to what the tree size was before you pruned - surges into spring growing mode. There is enough energy in the trunk and root system to support what was there when it went into dormancy in the autumn. The fact that a large proportion of its growth might have been removed in the winter, means that there is excess energy available, and this will materialise in new vigorous growths in the early spring season - usually at the expense of flower and fruit!
Any pruning in the winter season, should therefore be carried out as late as possible before bud break in order to avert this vigorous flush of unwanted growth. Most of these growths will be tall upright shoot, sprouting out from main branches. These are known as water shoots and have no use for fruiting, though one or two of them can be retained for future training if required. Hard pruning in the winter months will result in much excess growth - taking away much of the energy required for flower and fruit production.