Miners typically include the highest-paying transactions first, naturally. If there is a consistent backlog of transactions waiting to be included, low-fee or no-fee transactions may have to wait a long time, perhaps indefinitely. This creates fee pressure - at least in a scenario where Bitcoin clients are intelligent about fees and are able to add fees to pending transactions so that block space is effectively auctioned out. This is not necessarily today's scenario, but Bitcoin is working towards it.

But if blocks are not consistently full and there's no significant backlog, there is room for cheap transactions. This has been the case up to now. If the block size limit were to be increased, the assumption goes, there would again be room for cheap transactions - until they fill up again.

In practice, we don't know if miners would actually include cheap transactions if it makes their blocks significantly bigger. Their incentives seem to be against it, since bigger blocks have higher orphan rates.