Currently, the wasm heap is capped at 4GiB so we cannot expand it further. Once wasmtime stabilises support for wasm64, then we could allow heaps to grow beyond 4GiB as well. However, then we also have to worry about how does a canister with a ton of heap upgrades itself (because we currently wipe the heap on upgrades). So supporting wasm64 will open other issues that also need to be addressed.