There is no relevant decreasing of the search space.
If the difficulty has recently been adjusted (it is adjusted every 2016 blocks, which is approximately every 2 weeks), then the average time until a block is solved is 10 minutes. It doesn't matter if its been 3 hours since the last block, or if 10 blocks were all just solved in the past 15 seconds. The average time until the current block will be solved is 10 minutes.
If blocks are solved every 10 minutes on average, then shouldn't the average answer to the question "How long do I need to wait until the next block is solved?" be 5 minutes?