Why does infer Quartus so many M10k blocks in this case?

Daixiwen: Thanks a lot for the detailed explanation and the pointers. Finally the behaviour observed makes sence.

Unfortunately documentation is very sparse regarding this issue, in some older PDF I found the required hint - the maximum depth can be passed by attribute:

type mem_array is array (0 to mem_size-1) of std_logic_vector (dwidth-1 downto 0);

attribute max_depth : integer;

attribute max_depth of ram : variable is 1024;