Hi there a.p.
well at least I know the true reality thanks 
.
Still I haven't found a way to prove that RDM files are sparse files and how to investigate them on ESXi.
Normally I would that as explained here
Creating sparse files on Linux hosts with dd
but ESXi uses busybox and hence you don't have all the full options.
Is there any other way to check that indeed such file are sparse files? And how to access its metadata?