memory issues on a native c++ app on windows

Question

I'm investigating a bad_alloc crashes for a multithreaded native cpp app, from WinDbg it's clearly happening on allocating large object on heap (mostly basic_string ctor or some array allocation with new operator). From the !address -summary and memory analysis from DebugDiag it's seems like app memory usage is very high but heap size is still very small (around 70 MB).

LFH Key                   : 0x233116ff
Termination on corruption : ENABLED
  Heap     Flags   Reserv  Commit  Virt   Free  List   UCR  Virt  Lock  Fast 
                    (k)     (k)    (k)     (k) length      blocks cont. heap 
-----------------------------------------------------------------------------
05f60000 00000002   68964  56804  68964   8411 37570    13    2  29701      
    External fragmentation  14 % (37570 free blocks)
072a0000 00001002      60      4     60      2     1     1    0      0      
096f0000 00001002      60      4     60      2     1     1    0      0      
1f430000 00001002      60      4     60      2     1     1    0      0      
-----------------------------------------------------------------------------

I want to dig deep into the memory from the usage table and find out the cause of higher memory allocation, any suggestion on how to proceed further?