Why compiled Rust wasm binaries are 2-3 times bigger than Motoko ones?
One overview recently published: Collection libraries | canister-profiling
Most likely because they are tailored for different purposes. Rust is a general-purpose language, while Motoko is custom-built for the IC. Rust projects have a lot more dependencies than Motoko projects, and all these layers cannot be optimised away all that easily
I did not find information about it, however with Rust containers you have several options to optimize the size. The difference can be significant in footprint.