It's unfortunately very easy to exceed the 64 KB cart size limit in Rust without some extra steps. I explored some different techniques here for reducing the size of the example snake tutorial. I got it down from 140 KB to 7 KB using a few different techniques, but just using `wasm-opt -Oz --strip-producers --dce --zero-filled-memory --strip-debug` is enough to get it down to ~22 KB.
Some folks are working on incorporating `wasm-opt` into wasm4's rust project template, as it already is for the others, but for now you need to invoke it manually (or write a wrapper script like this one).