Compound page is just an implementation detail how page-table data structure represents the pages larger than 4K. And this is reasonable to me because in order

Compound page is just an implementation detail how page-table data structure represents the pages larger than 4K. And this is reasonable to me because in order to create a mapping for 2MB page, kernel needs to group 512 consecutive 4K pages, and per my understanding this is exactly what the kernel will do. For a 1GB page it will have to group 512 page-tables, each containing 512 4K entries. There is no separate page-table data structure that the kernel will maintain for 2MB or 1GB pages. There's only one. However, you're right that multiple processes maintaining separate page-tables where each is containing mappings to the same physical memory regions can be wasteful. Still, due to how huge pages are represented in the page-table data structure, I don't see what difference would it make to switch to 2MB or 1GB page, unless you're running a recentish kernel (~3 years) that has been specifically optimized for exact this case, e.g. to reduce the memory footprint of huge-page representation in page-tables. Specifically, a single 2MB huge page is now represented by 8 page entries (512 bytes) instead of 512 page entries (2MB). And there was another patch proposing to cut 8 page entries to only 2 page entries. In any case, that's a dramatic cut of memory footprint when compared to how it used be and which is why I believe that this is the actual reason why OP sees the difference.