Spark's tungsten memory model usually tries to allocate memory by one `page` each time and allocated by long[pageSizeBytes/8] in HeapMemoryAllocator.allocate.
Remember that java long array needs extra object header (usually 16 bytes in 64bit system), so the really bytes allocated is pageSize+16.
Assume that the G1HeapRegionSize is 4M and pageSizeBytes is 4M as well. Since every time we need to allocate 4M+16byte memory, so two regions are used with one region only occupies 16byte. Then there are about 50% memory waste.
It can happenes under different combinations of G1HeapRegionSize (varies from 1M to 32M) and pageSizeBytes (varies from 1M to 64M).