The Rust toolchain by default produces a lot of debug information for each crate being compiled and the language relies heavily on monomorphization (instantiating a different function/type for each combination of generics) which translates to generally having duplicated functionality where the only difference in the object code is the different types. These two things combined, along with large dependency trees and that "features" for conditional compilation cause the same crate to be considered different across different workspaces (meaning the rlibs that were created for one can't be reused for another), means that the target directory for Rust projects can quickly grow. That, + clippy not having stabilized an already implemented process to automatically delete files from the target directory when they are unlikely to be needed again (which can't be 100% accurate, it uses heuristics because a developer could always potentially need an exact rlib from 6 months ago, it is just unlikely) means that you can expect that under the default configuration your target directory ends up being huge after a short period of time.

A lot of those things are in different stages of being fixed. I would personally want the default for generated debug information to be changed. cargo gc will help with running out of disk space. Better cross-project reuse of existing rlibs could be accomplished with some better tracking of optional features that can be unified (think having `serde` with feature `derive` enabled in one project, and having it without it in another: features are supposed to be purely additive, so you could reuse the earlier rlib on the later, but in practice people (ab?)use features in non-additive ways so it isn't straighforward to do something like that by default). Tools like hakari lets you create workspace-unified Cargo.toml to emulate this behavior.