Because you're removing buds, before the energy accumulated during the autumn is spent...
Imagine you have 100% energy to distribute for 100 buds. Each bud will receive 1% of that energy right? If you cut back and reduce to 10 buds prior to leafing out, all that energy will be fuelled to those 10 buds. Each bud will now have 10% of energy. So they will grow stronger (=longer internodes, larger leaves).
If you wait for the spring push, and then prune back, the tree now has less than 100% energy cause it already spent some growing all those leaves...
The above is just an analogy of course, but I think it will make you understand the concept.