Mesh's fragmentation protection is probabilistic (but highly effective) and as fragmentation grows, the likelihood of it being able to perform compaction also grows. Intuitively, if you have lots of sparsely allocated pages, Mesh is almost certainly going to be able to find many pairs of pages that it can "mesh" together into a single physical page. If the pages are too dense, then Mesh becomes ineffective, but that's exactly the point where there's little opportunity to squeeze out memory savings and the pages aren't really fragmented.