Pruning reduces historic data, it doesn't reduce the unspent transactions outputs— they could be spent at any time so you need a ultimately trusted copy of them in order to validate a block you receive (and prevent theft and inflation). Part of the problem with people cramming data in 1e-8 value txouts— which are completely and forever unspendable, but not in a way which can be machine detected— is that it increases the size of the data which cannot ever be pruned.