Thank you, Haida, for asking the question and apologies for the late response as I was sick last week.
In Motoko, you can use Enhanced Orthogonal Persistence with the --enhanced-orthogonal-persistence compiler flag. This enables Wasm64 and super-fast upgrades of constant time regardless of the heap size, without any use of stable memory. (Just in case you are interested in the technical aspects, here is a good reference: Smarter Contract Upgrades with Orthogonal Persistence | Proceedings of the 16th ACM SIGPLAN International Workshop on Virtual Machines and Intermediate Languages)
For Wasm64, the IC currently supports 6 GB and we intend to extend this further to 500 GB in future, to the same capacity as stable memory.
We also plan to make enhanced orthogonal persistence (EOP) the default for Motoko very soon.
The current default, classical persistence only supports maximum 2GB of stable objects (reachable through stable variables). And even then, several data structures (with sharing of immutable objects or structures with long pointer traces) do not scale at all and even crash at a few kilo- or megabytes.
Please let me know if I can provide more information.