I heard the default memory for instances was 32MB, which is far more than any contract I have written will need. The size of each one limits the number of contracts that can be held in memory via the Instance LRUCache.
It seems there is a fair bit of control of the memory regions that you can set up in an instance.
You can control min and max memory in the descriptor: https://docs.rs/wasmer-runtime/0.4.2/wasmer_runtime/wasm/struct.MemoryDescriptor.html
Measure the default memory usage (both of the wasm memory pages and the instance overhead (globals, wasm bytecode, etc). Then experiment on optimizing it, and experiment with the LRUCache. Definitely document this.