Why does the memory consumption of DMRG far exceed the sum of MPS and MPO by several times?

Yes we are finding this issue too - Julia accumulates memory during runtime. You can see some benchmarking of this issue on in this post Memory usage in DMRG with Julia 1.x - #7 by mtfishman. We are working on creating a memory buffer system to minimize this problem and improve performance. I think the recommended solution is still the --heap-size-hint flag but you can follow Matt’s thread for updates.