In response to receiving a write request directed to a particular logical block of a storage object, a page of free space (sufficient to accommodate the payload of the write request, but smaller in size than the logical block) of a particular extent that has been selected to store contents of the logical block is allocated. The current size of the extent is smaller than the combined sizes of logical blocks that are mapped to the extent. The page is modified in accordance with a payload indicated in the write request. In response to a subsequent write request directed to the particular extent, a determination is made that the particular extent would violate a free space threshold criterion if the payload of the write request were accommodated, and an extent expansion operation is initiated.