So requesting a 10 byte chunk of virtual address space might actually lead to a whole 4K page being reserved and mapped. Since the physical memory within the page must be contiguous this can lead to a whole 4K of virtual address space being 'blocked off'.
Multiple small allocations can be placed within one page (and good memory managers will try to achieve this) but none the less address space has in effect been reserved over and above what was strictly required. Consider the allocation of a single byte at the start f a page, then 4K - 2 bytes followed by another single byte. This occupies (efficiently) the whole page.
Consider if the middle allocation is no longer necessary, and is thus freed. Until the 'top' and 'tail' values are either freed or moved elsewhere a gap has been created in the virtual address space which can only be filled by something < 4K-1 bytes in size.If sufficient of these sorts of things occur the contiguous regions in virtual address space shrink much faster than the actual total really used memory does.